Document:

<PAGE>   1

                                                                   EXHIBIT 10.15

                         E9-1-1 CLEARINGHOUSE SERVICES

                                   AGREEMENT

                          BETWEEN SCC COMMUNICATIONS,

                                    INC. AND

                             ADVANCED TELCOM GROUP,

                                      INC.

--------------------
[*] Information redacted pursuant to a confidential treatment request
    throughout this exhibit.
<PAGE>   2

                                     E9-1-1

                                 CLEARINGHOUSE

                                    SERVICES

                                MASTER CONTRACT

                              TERMS AND CONDITIONS

                                   [SCC LOGO]

                               6285 LOOKOUT ROAD
                      BOULDER, COLORADO, U.S.A. 80301-3343
                                 (303) 581-5600

<PAGE>   3

                               TABLE OF CONTENTS

<TABLE>
<S>                                                                       <C>
RECITALS .................................................................  4

1.  DEFINITIONS ..........................................................  4
1.1   Services ...........................................................  4
1.2   New Services .......................................................  4
1.3   Add-On Services ....................................................  4
1.4   Modification Order .................................................  5
1.5   Project ............................................................  5
1.6   PSAP ...............................................................  5
1.7   Base Record ........................................................  5
1.8   ATGI Affiliate .....................................................  5
1.9   Subscriber .........................................................  5

2.  STATEMENT OF WORK ....................................................  6
2.1   Responsibilities ...................................................  6
2.2   Project Implementation Plan ........................................  6
2.3   Project Management .................................................  6
2.4   Progress Meetings ..................................................  6
2.5   Training ...........................................................  7
2.6   New Services .......................................................  7
2.7   Add-On Orders ......................................................  7
2.8   Modification Orders ................................................  7
2.9   Letter Of Agency ...................................................  7
2.10  Electronic Exchange ................................................  8
2.11  Uniformity .........................................................  8

3.  ATGI RESPONSIBILITIES ................................................  8

4.  TERM AND TERMINATION .................................................  8
4.1   Term ...............................................................  8
4.2   Termination.........................................................  8
4.3   Orderly Transition .................................................  9
</TABLE>

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                         SCC PROPRIETARY & CONFIDENTIAL

<PAGE>   4

<TABLE>
<S>                                                                          <C>
5.  PRICE AND PAYMENT .....................................................  9
5.1   Services Fees .......................................................  9
5.2   Audits .............................................................. 10
5.3   Set-Off ............................................................. 11
5.4   Disputed Invoices.................................................... 11
5.5   Taxes ............................................................... 11
5.6   Other Charges ....................................................... 11

6.  LICENSE AND DEVELOPED INFORMATION ..................................... 12
6.1   Licenses ............................................................ 12
6.2   Developed Information - Definitions ................................. 12
6.3   Developed Information - Rights ...................................... 13

7.  SYSTEM SECURITY ....................................................... 13
7.1   SCC and ATGI Data ................................................... 13
7.2   Address Verification ................................................ 13

8.  CONFIDENTIALITY ....................................................... 14
8.1   Confidential Information ............................................ 14
8.2   Employees ........................................................... 14
8.3   Exceptions .......................................................... 14
8.4   Ownership of Confidential Information ............................... 15

9.  WARRANTIES ............................................................ 15
9.1   Warranty By SCC ..................................................... 15
9.2   Warranty By ATGI .................................................... 16
9.3   Exclusion ........................................................... 16

10. INDEMNIFICATION ....................................................... 17
10.1  General Indemnification ............................................. 17
10.2  Intellectual Property Indemnification ............................... 17
10.3  ATGI Immunity ....................................................... 18

11. LIMITATION OF LIABILITY ............................................... 18
11.1  Consequential Damages ............................................... 18
</TABLE>

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                         SCC PROPRIETARY & CONFIDENTIAL

<PAGE>   5

<TABLE>
<S>                                                                                 <C>
11.2  Maximum Liability ........................................................... 18

12. INSURANCE ..................................................................... 19
12.1  Insurance Coverage .......................................................... 19
12.2  Evidence of Coverage ........................................................ 19

13. RECURRING SERVICE ERROR/LIQUIDATED DAMAGES..................................... 19
13.1  Recurring Service Error ..................................................... 19
13.2  Liquidated Damages .......................................................... 19

14. GENERAL PROVISIONS ............................................................ 19
14.1  Advertising and Publicity ................................................... 19
14.2  Assignment .................................................................. 20
14.3  Authority ................................................................... 21
14.4  Company Rules ............................................................... 21
14.5  Escalation Procedures ....................................................... 21
14.6  Force Majeure ............................................................... 21
14.7  Governing Law ............................................................... 21
14.8  Independent Contractors ..................................................... 21
14.9  Joint Work Product .......................................................... 22
14.10 Laws, Regulations, Permits .................................................. 22
14.11 Notices ..................................................................... 22
14.12 Remedies .................................................................... 23
14.13 Non-Waiver .................................................................. 23
14.14 Severability ................................................................ 23
14.15 Binding Effect .............................................................. 23

15. ENTIRE AGREEMENT .............................................................. 24

EXHIBIT A         STATEMENT OF WORK
EXHIBIT B         FEES AND PAYMENT SCHEDULE
EXHIBIT C         MODIFICATION ORDER
EXHIBIT D         ESCALATION PROCEDURES
EXHIBIT E         LETTER OF AGENCY
</TABLE>

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                         SCC PROPRIETARY & CONFIDENTIAL

<PAGE>   6

                    CLEARINGSHOUSE SERVICES AGREEMENT BETWEEN
                        SCC COMMUNICATIONS CORP. AND ATGI

         THIS AGREEMENT is made this __ day of May, 1999, (the "Effective Date")
between SCC Communications Corporation, ("SCC") a Delaware Corporation, with its
principal offices located at 6285 Lookout Road, Boulder, Colorado 80301, and
Advanced TelCom Group, Inc. ("ATGI"), a Delaware corporation having its
corporate offices located at 100 Stony Point Road, Suite 130, Santa Rosa,
California, 95401, who are collectively referred to herein as "the Parties" or
individually as "Party."

                                    RECITALS

         WHEREAS, ATGI wishes to procure from SCC the enhanced 9-1-1 data base
management services described herein; and

         WHEREAS, SCC wishes to provide such services (defined herein as
"Services").

         NOW, THEREFORE, in consideration of the mutual promises and covenants
set forth herein and for good and valuable consideration, the sufficiency of
which is hereby acknowledged, SCC and ATGI, (the "Parties"), agree as follows:

                                 1. DEFINITIONS

1.1      SERVICES

         "Services" means the services described in the "Statement of Work"
("SOW"), attached as Exhibit A, which Services are intended to enable ATGI to
implement and maintain an E9-1-1 system for all of ATGI's Subscribers (as
defined herein). "Services" shall also include those New Services and Add-On
Services that are added to this Agreement pursuant to a duly executed
Modification Order.

1.2      NEW SERVICES

         "New Service" means those services developed by SCC in its sole
discretion which modify, improve or add functionality, of any level, kind or
version, to the Services and which are offered to and accepted by ATGI pursuant
to subsequent agreement of the Parties.

1.3      ADD-ON SERVICES

         "Add-On Service" means services that are not Services or New Services
initially contemplated herein, but which are developed by SCC as a result of
ATGI's explicit request for specific modifications, improvements or additional
functionality to the Services or New Services

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                         SCC PROPRIETARY & CONFIDENTIAL

<PAGE>   7

and which are added to same pursuant to subsequent agreement of the Parties.

1.4      MODIFICATION ORDER

         "Modification Order" means a document that, by its formal execution,
obligates the Parties with respect to the addition, or modification, of New
Services or Add-On Services described in such document. Such Modification Order
shall be subject to the terms and conditions hereof, shall be prepared in
substantially the same form as that set forth in Exhibit C and shall become
effective only when fully executed by the authorized representatives of each
Party as evidenced thereon.

1.5      PROJECT

         "Project" means the undertaking of the tasks and duties necessary to
implement and provide the Services.

1.6      PSAP

         "PSAP" means Public Safety Answering Point as that phrase is commonly
known in the telecommunications industry.

1.7      BASE RECORD

         "Base Record" means a database record that includes the name, address
or address equivalent, and the telephone number of a Subscriber.

1.8      ATGI AFFILIATE

          "ATGI Affiliate" means any entity that directly, or indirectly through
one or more intermediaries, controls or is controlled by or is under common
control with ATGI, as well as any successor to ATGI, whether by change of name,
dissolution, merger, consolidation, reorganization or otherwise.

1.9      SUBSCRIBER

          "Subscriber" means each ATGI customer whose address falls within the
municipal, county or other jurisdictional boundary served by a PSAP.

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                         SCC PROPRIETARY & CONFIDENTIAL

<PAGE>   8

                             2. STATEMENT OF WORK

2.1      RESPONSIBILITIES

         The Statement of Work attached hereto, incorporated herein and marked
as "Exhibit A" is a complete description of the Services. The Parties shall
perform their respective duties described therein during the Project. The
Parties further agree to jointly develop, and in good faith negotiate, a
detailed statement of work for any Add-On Services which shall be appended to
and incorporated into the appropriate Modification Order, which itself shall be
incorporated into this Agreement. All requests for additional services,
conversion of additional records, training, and enhancements shall be
coordinated through the Project Managers and handled in accordance with this
Agreement. All requests by either Party shall be considered and negotiated in
good faith and incorporated into this Agreement if mutually agreed to in
writing.

         SCC will, at all times relevant hereto, maintain a sufficient number of
employees in its discretion to adequately perform the Services, and if
applicable, New Services and Add-On Services.

2.2      PROJECT IMPLEMENTATION PLAN

         A fully detailed Implementation Plan shall be negotiated by the Parties
as specified in the SOW, which Implementation Plan shall define the specific
tasks, responsibilities, performance dates of each Party, the specific services
to be provided, performance requirements, and objectives for the provisioning of
the Services according to Exhibit A. Any subsequent changes to the
Implementation Plan shall be mutually agreed to in writing.

2.3      PROJECT MANAGEMENT

         SCC and ATGI shall each designate an individual, or individuals as may
be changed from time to time, as the Party's respective Project Manager, who
will act as the primary interface between the Parties. The Project Managers
shall be responsible for insuring the continuity of communications between the
Parties as the Project proceeds. The Escalation Procedures contained in "Exhibit
D" attached hereto and incorporated herein shall be included as part of the
Statement of Work.

2.4      PROGRESS MEETINGS

         On a periodic basis, the Project Managers shall meet in order for the
Parties to inform each other of the status of the Project, of each Party's
respective tasks and responsibilities, as well as ATGI's future roll-out plans.
At ATGI's request, each Party shall provide to the other written status reports
on the work being performed. ATGI may elect to forego all or some of such
meetings and may direct SCC to provide it with periodic written or oral reports
on the status of the Statement of Work.
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                         SCC PROPRIETARY & CONFIDENTIAL

<PAGE>   9

2.5      TRAINING

         In connection with the Implementation Plan described in Exhibit A, SCC
shall provide training to ATGI's personnel. The Parties shall subsequently
negotiate the dates and times of such training, and unless otherwise agreed,
training shall take place in Boulder, Colorado at SCC's offices. If SCC
ultimately agrees to provide training at any other location, SCC shall be
reimbursed for its costs of travel, which costs shall not exceed the allowances
for same as set forth in ATGI's then-current, standard travel policy. Once
scheduled, training classes may be canceled by ATGI without penalty upon ten
(10) business days prior written notice to SCC. For cancellations with less than
ten (10) business days prior written notice, ATGI shall be liable to SCC for
SCC's time and its cost of travel and training materials actually incurred by
SCC in preparation for the course only for those costs not otherwise recoverable
by SCC.

2.6      NEW SERVICES

         At SCC's sole discretion, New Services will be periodically offered to
ATGI by SCC hereunder based on additional products SCC develops, also at SCC's
sole discretion. All requests for New Services shall be coordinated through the
Project Managers and handled in accordance with the terms of this Agreement.

2.7      ADD-ON ORDERS

         ATGI may request in writing specific modifications or improvements to
the Services at any time. SCC shall consider each in good faith and provide to
ATGI a quotation to include at a minimum, the description of the Add-On Service,
date of availability, proposed implementation schedule, initial charges and/or
additional recurring charges, and other fees as are reasonably required. SCC and
ATGI shall negotiate in good faith the final terms and an additional statement
of work.

2.8      MODIFICATION ORDERS

         If ATGI elects to purchase from SCC New Services or Add-On Services,
ATGI will deliver to SCC a completed Modification Order form substantially
similar to that which is set forth in Exhibit C. The parties will negotiate in
good faith the terms and conditions relating to the provision of such services,
which shall include at a minimum: (1) a description of the requested service;
(2) a projected date of its availability; (3) a proposed implementation
schedule; (4) the initial non-recurring charges and/or additional recurring
charges associated with the service; and (5) any other information deemed
appropriate by the Parties. Modification Orders will not be effective or binding
upon the parties unless signed by an authorized individual from ATGI.

2.9      LETTER OF AGENCY

         Parties agree that a Letter of Agency ("LOA") has already been executed
by the Parties and is attached to this Agreement as Exhibit E, which letter will
enable SCC to perform the Services as a limited agent for ATGI. The LOA shall:
(1) not be released by SCC or used except as
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                         SCC PROPRIETARY & CONFIDENTIAL
<PAGE>   10
authorized by this Agreement; (2) be deemed revoked upon termination of this
Agreement or delivery to SCC of written notice of ATGI's election in this
regard; and (3) be returned to ATGI upon ATGI's request, upon termination of
this Agreement or upon delivery of a revocation notice as described herein.

2.10    ELECTRONIC EXCHANGE

        Except as may otherwise be provided herein, the Parties will use and
implement electronic automation procedures to facilitate communication,
coordination, maintenance, and management of the Services. In addition, if
either party modifies any of the software, protocols, systems, data
communications or other crucial aspects which relate to the provision of SCC's
Services or New Services, the party initiating such modification shall provide
to the other party written notice of such modification not less than ninety
(90) days prior to such modification, and the other party shall have not less
than one hundred eighty (180) days to implement the changes necessary to
accommodate such modification. Responsibility for payment of any additional
costs for the non-initiating Party in order to accommodate such change shall be
negotiated by the Parties in good faith.

2.11    UNIFORMITY

        The Services shall not be either province, state, or regionally
specific unless it is legally mandated by the legal governing body.

                            3. ATGI RESPONSIBILITIES

        ATGI'S contractual obligations and responsibilities are those stated in
this Agreement and its Exhibits, as may be amended by mutual written agreement
of the Parties.

                            4. TERM AND TERMINATION

4.1     TERM

        The term of this Agreement shall begin upon the Effective Date as first
stated above and continue for a period of three (3) years thereafter unless
earlier terminated under the terms of this Agreement ("Initial Term"). Following
the Initial Term, the Agreement shall automatically renew for continuous one
(1) year terms unless terminated at the end of a renewal term upon no less than
180 days advance written notification by the terminating Party.

4.2     TERMINATION

        Either party may terminate this Agreement in the event that the other
Party is in default under the terms or conditions of this Agreement including,
but not limited to, any amendments thereto or any terms and conditions
contained in the SOW, any other authorized exhibits or

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                         SCC PROPRIETARY & CONFIDENTIAL
<PAGE>   11
SCC/ATGI                       Confidential                     Page 9 05/14/99
----------------------------------------------------------------------

amendments thereto. The non-defaulting Party shall promptly provide written
notice of default to the defaulting Party. Except for a default alleging
non-payment, the defaulting Party shall have thirty (30) days from the
effective date of the written notice to cure such default or to provide a plan
to cure same which plan must be accepted in writing by the non-defaulting
Party. For a default for non-payment, the defaulting Party shall have fifteen
(15) days in which to cure such default. If a timely cure is not made as
provided herein, then this Agreement may be terminated at the option of the
non-defaulting Party.

4.3  ORDERLY TRANSITION

     Upon expiration or proper termination hereof, SCC will cooperate in the
orderly transition of the Services to ATGI or its agent and will make reasonable
efforts to minimize any disruption of ATGI's E9-1-1 service. At such time, and
with respect to the ATGI-relevant data and databases that SCC legally controls
hereunder, SCC will deliver to ATGI such data contained in such databases, and
other information that is necessary to allow ATGI to be substituted for SCC in
this regard; provided, however, that SCC shall not be obligated to do so if such
delivery or other performance would violate the confidentiality provisions of
this Agreement, result in a breach of contract to which SCC is a party as of the
Effective Date hereof, violate any other law or private right, convey to ATGI
any intellectual property or other right to which ATGI is otherwise un-entitled
hereunder, or otherwise unreasonably subject SCC to civil or criminal
prosecution. Such data and information shall include but not be limited to ATGI
Results (as defined herein) along with the data needed to perform said Services,
all in a readable file or files of reasonable structure and convertible with
commercially available software, which data and information shall be current as
of the date of such transition.

      If ATGI requests that SCC continue, beyond the effective date of
expiration or termination, to provide any portion or all of the Services that
SCC would not otherwise be contractually obligated to provide under the terms
and conditions of this Agreement, ATGI shall compensate SCC for such Services
at the same rate as otherwise would be applicable under this Agreement for such
Services for so long as such Services or portions thereof are being provided.

                            5.    PRICE AND PAYMENT

5.1   SERVICES FEES

      The Fee and Payment Schedule, attached hereto, marked as Exhibit B and
incorporated herein, sets forth the charges and fees payable by ATGI to SCC for
the Services.

      5.1.2 ATGI's OPTION FOR "PRICING ADJUSTMENT."

      ATGI will initially pay the rates pertaining to the MBFs as set forth in
Exhibit B based on the number of ATGI's actual Base Records. Unless ATGI
exercises its option described herein, ATGI will, for the duration of this
Agreement, pay such rates based on the number of ATGI's actual Base Records. If
ATGI exercises such option, the MBFs shall be determined in the

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                         SCC PROPRIETARY & CONFIDENTIAL
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SCC/ATGI                    Confidential                      Page 10    5/14/99
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manner set forth below.

     At ATGI's sole option, ATGI may invoke a "Pricing Adjustment", as described
below, by electing to pay MBFs at the rates set forth in Exhibit B based on the
number of ATGI's projected Base Records instead of the number of ATGI's actual
Base Records. If ATGI exercises such option, then no later than sixty (60) days
prior to the effective date of the Pricing Adjustment, ATGI will provide SCC
with ATGI's forecast of ATGI's projected number of Base Records which ATGI
expects will exist one (1) year following the Pricing Adjustment. Such forecast
shall be in writing and delivered to SCC's Vice President of Sales pursuant to
the Notice requirements of this Agreement. ATGI's Projected Base Records shall
be reasonable and shall be consistent with ATGI's overall business plans, and
SCC shall be given a reasonable opportunity to verify this criteria for
reasonableness and consistency.

     If ATGI exercises its option to invoke the Pricing Adjustment, and, if at
the end of the fifth (5th) month following the Pricing Adjustment, ATGI's actual
Base Records are not equal to or greater than [*] of the number of Projected
Base Records, then: (a) ATGI will thereafter pay the MBFs based on the actual
number of ATGI's actual Base Records, and (b) ATGI will be invoiced
retroactively for the preceding five (5) months in the amount of the difference
between the MBFs based on the Projected Base Records and the MBFs based on the
actual number of Base Records.

     If ATGI exercises its option to invoke the Pricing Adjustment, and, if at
the end of the fifth (5th) month following the Pricing Adjustment, ATGI's actual
Base Records are equal to or greater than [*] of the total number of the
Projected Base Records, ATGI shall continue to pay the MBFs based on the number
of Projected Base Records instead of the number of actual Base Records; provided
that if the number of ATGI's actual Base Records exceed ATGI's Projected Base
Records, ATGI will pay based on the number of actual Base Records. Regardless of
the outcome of the exercise by ATGI of its option, the rate at which ATGI shall
be obligated to pay MBFs, after the first (1st) year following the Pricing
Adjustment and beyond, will be based on actual Base Records.

5.2  AUDITS

     At its sole expense, SCC shall maintain complete and accurate books and
records with respect to the Project and the Services, and said books and
records shall be maintained in accordance with generally accepted accounting
principles. Each Party shall have the right to audit the books and records of
the other that relate specifically to the calculation of the number of
Subscribers and related fees. In addition, ATGI shall have the right to audit
the books and records of SCC that relate to SCC's data-protection and security
obligations hereunder. Such audits may be performed at the premises of the
other Party, shall be limited to once per calendar year (except for good cause
reasonably demonstrated) and shall only be allowed with not less than fifteen
(15) days advance written notice to the audited Party. Each Party will be
responsible for its own costs related to such audits. "Undisputed" invoices, as
defined herein, which are not adjusted within eighteen (18) months of the date
of such invoice shall be deemed accepted by the Parties, regardless of whether
such invoice was included in a prior or subsequent audit.

--------------------
[*] Information redacted pursuant to a confidential treatment request
    throughout this exhibit.

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                         SCC PROPRIETARY & CONFIDENTIAL
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SCC/ATGI                           Confidential                          Page 11
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5.3  SET-OFF

     All legitimate and reasonable claims for money due or to become due from
ATGI shall be subject to deduction or set-off by ATGI by reason of any such
amounts due ATGI pursuant to this Agreement; however no set-off or
"cross-collateralization" shall be permitted hereunder in connection with
amounts that may be in dispute or owed pursuant to any other transaction or
agreement between the Parties.

5.4  DISPUTED INVOICES

     In the event of a disputed invoice, ATGI shall notify SCC within twenty
(20) days of the receipt of any such invoice and identify the nature of the
dispute or inaccuracy. ATGI shall pay any undisputed amounts set forth in said
invoice in accordance with the provisions hereof pertaining to normal payment
of invoices. Both parties shall in good faith investigate and attempt to
resolve the outstanding disputed amount, and once resolved, any payments owed
shall be promptly paid.

5.5  TAXES

     Fees and charges payable hereunder shall not include any sales, use,
excise, transaction or other similar taxes levied against or upon the
furnishing or receipt of Services. Further, the amounts payable by ATGI
hereunder shall not include any federal, state and local taxes, taxes that are
based on SCC's net or gross receipts, franchise taxes or other taxes based on
SCC's corporate existence or status, personal property taxes on licensed
software and taxes that may be due in whole or in part because of any failure
by SCC or its agents to file any return or information required by law, rule or
regulation. If any sales or other taxes related to SCC's provision of Services
are payable or mandated by current or prospective application of law, they
shall be separately stated on the monthly invoice to ATGI, and ATGI shall be
responsible for paying same in accordance with the terms and conditions set
forth in this Section 5 for payment of invoices. ATGI shall reimburse SCC for
any penalties or interest actually levied upon SCC with respect to such taxes
only if ATGI's acts or omissions solely caused such penalty or interest to be
levied.

5.6  OTHER CHARGES

     Charges for miscellaneous services not contemplated herein will be offered
to ATGI on a time-and-materials basis using SCC's then-current rates for same.
In the event ATGI requests that SCC provide such services, SCC shall first
furnish ATGI with an estimate of the amount of such charges (the "Other
Charges"), which, upon written agreement of the Parties, may include travel and
other related expenses, and shall specify a reasonable "not-to-exceed" billable
amount. Billings of Other Charges for the requested services shall not exceed
the specified amount without ATGI's prior written approval. Other Charges shall
be billed in arrears, after provision of the relevant services, and ATGI shall
pay Other Charges within thirty (30) days of ATGI's receipt of invoices related
to such services. SCC shall be responsible for the normal, non-fixed costs and
fees for acquiring MSAGs ("Master Street Address Guides") in an amount not to

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                         SCC PROPRIETARY & CONFIDENTIAL

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SCC/ATGI                     Confidential                      Page 12  5/14/99
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exceed [*] per MSAG, as described in Exhibit A. ATGI shall be responsible for
paying the amount, if any, in excess thereof.

                      6.  LICENSE AND DEVELOPED INFORMATION

6.1  LICENSES

     Except as may otherwise be set forth herein or in a fully executed
Modification Order, nothing in this Agreement shall be construed to grant ATGI
any right, title or interest in or license to any SCC Furnished Technology (as
defined herein), SCC hardware or software products or services used in the
provision of Services, New Services, any other SCC proprietary information or
intellectual property, or SCC-owned Proprietary Rights (as defined herein).
Nothing in this Agreement shall be construed to grant SCC any right, title or
interest in ATGI Furnished Technology (as defined herein), ATGI proprietary
information, intellectual property, any data being provided to SCC by ATGI for
use in the provision of Services, New Services, Add-On Services or ATGI-owned
Proprietary Rights (as defined herein).

6.2  DEVELOPED INFORMATION - DEFINITIONS

     6.2.1  PROPRIETARY RIGHT

     "Proprietary Right" means any patent, copyright, trade secret, trademark
or other intellectual property right that is protected or protectable under the
laws of any governmental authority having jurisdiction.

     6.2.2  ATGI FURNISHED TECHNOLOGY

     "ATGI Furnished Technology" means any design, specification, know-how,
computer program, computer software, computer hardware, device, technique,
algorithm, method, procedure, discovery or invention, whether or not reduced to
practice, or enhancement, improvement or derivative works thereof that (1) is
protected or protectable under any Proprietary Right, (2) is owned or
controlled (by license or otherwise) by ATGI or any of its affiliates, and (3)
is furnished or to be furnished by ATGI or any of its affiliates to SCC under
this Agreement. ATGI represents that any of its employees or agents who have or
may have a right of interest in or to ATGI Furnished Technology have executed
proper waivers granting all right, title and interest in same to ATGI.

     6.2.3  SCC FURNISHED TECHNOLOGY

     "SCC Furnished Technology" means any design, specification, know-how,
computer program, computer software, computer hardware, device, technique,
algorithm, method, procedure, discovery or invention, whether or not reduced to
practice, or enhancements, improvements or derivative works thereof that (1) is
protected or protectable under any Proprietary Right, (2) is owned or
controlled (by license or otherwise) by SCC or any of its affiliates, and (3)
is furnished or to be furnished by SCC or any of its affiliates to ATGI under
this Agreement. SCC represents that any of its employees or agents who have or
may have a right or interest in or to SCC Furnished Technology, or those who
are performing Services hereunder,
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                         SCC PROPRIETARY & CONFIDENTIAL

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SCC/ATGI                    Confidential                        Page 13  5/14/99
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have executed appropriate waivers granting all right, title and interest in
same to SCC.

     6.2.4 ATGI RESULTS

     "ATGI Results" means any data that: (a) is not data that comprises any or
all of the Services, New Services or Add-On Services (as defined herein), and
(b) is not public information, and (c) is not SCC Furnished Technology or the
property of any third party, and (d) is ATGI-specific data.

6.3  DEVELOPED INFORMATION - RIGHTS

     6.3.1 ATGI RESERVATION OF RIGHTS

     ATGI reserves all of its right, title and interest in all ATGI Furnished
Technology and all Proprietary Rights in same.

     6.3.2 SCC RESERVATION OF RIGHTS

     SCC reserves all of its right, title and interest in all SCC Furnished
Technology and all Proprietary Rights in same.

     6.3.3 OWNERSHIP OF ATGI RESULTS

     ATGI will be the exclusive owner of all right, title and interest in the
ATGI Results and all Proprietary Rights in same. To the extent permitted under
the United States Copyright Act (17 U.S.C. Section 101 et seq. and any successor
statutes thereto), the ATGI Results will constitute "works made for hire" and
the ownership of such ATGI Results will vest in ATGI at the time they are
created. In any event, SCC hereby assigns and transfers, and promises to assign
and transfer, to ATGI all SCC's right, title and interest in the ATGI Results
and Proprietary Rights thereto.

                               7. SYSTEM SECURITY

7.1  SCC AND ATGI DATA

     SCC and ATGI acknowledge the importance of maintaining the security and
integrity of the systems and the data of each Party as a result of their access
to those systems and data. In order to protect the data and systems, each Party
shall strictly adhere to the nondisclosure and system security policies in the
performance of its own tasks and the services related to this Agreement. SCC
understands and acknowledges the confidential nature of the ATGI's telephone
subscriber information. SCC and its employees shall not disclose data to which
it has access under this Agreement to any person or entity without the prior
written consent of ATGI except as required in order to fulfill its obligations
under this Agreement.

7.2  ADDRESS VERIFICATION

     ATGI acknowledges that SCC may freely use and exchange, according to
standard industry practices, Master Street Address Guide ("MSAG") data with
other service providers, public safety agencies, etc., in order to establish
the proper jurisdiction and call management

-------------------------------------------------------------------------------
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                         SCC PROPRIETARY & CONFIDENTIAL

<PAGE>   16
SCC/ATGI                          CONFIDENTIAL                           Page 14
--------------------------------------------------------------------------------

services and as necessary for SCC to provide its Services, New Services and
Add-On Services without the disclosure of Confidential Information as required
under this Agreement.

                              8.   CONFIDENTIALITY

8.1  CONFIDENTIAL INFORMATION

     During the course of this Agreement, either Party may receive or have
access to confidential information of the other. "Confidential Information"
means any confidential, private, or proprietary information or data disclosed
by a Party (the "Disclosing Party") to the other Party (the "Recipient") under
or in contemplation of this Agreement which (a) if in tangible form or other
media that can be converted to readable form is clearly marked as Confidential,
proprietary, or private when disclosed, or (b) if oral, or visual, is
identified as Confidential, proprietary, or proprietary on disclosure. The
terms "Disclosing Party" and "Recipient" include each Party's corporate
affiliates that disclose or receive Confidential Information. The rights and
obligations of the Parties shall therefore also inure to such affiliates and
may be directly enforced by or against such affiliates. The Recipient
acknowledges the economic value of the Disclosing Party's Confidential
Information. The Recipient therefore, shall:

     (i)   use the Confidential Information only in connection with the
     Recipient's performance of its obligations or in exercising its rights
     under this Agreement;

     (ii)  restrict disclosure of the Confidential Information to employees of
     the Recipient and its affiliates with a "need to know" and not disclose it
     to any other person or entity without the prior written consent of the
     Disclosing Party;

     (iii) advise those employees who access the Confidential Information of
     their obligations with respect thereto; and

     (iv)  copy the Confidential Information only as necessary for those
     employees who are entitled to receive it and ensure that all
     confidentiality notices are reproduced in full on such copies.

8.2  EMPLOYEES

     For the purposes of this Agreement only, "employee" includes third parties
retained by the Parties for temporary administration, clerical, or programming
support. A "need to know" means that the employee requires the Confidential
Information to perform his or her responsibilities in connection with this
Agreement.

8.3  EXCEPTIONS

     The obligations of this section shall not apply to any Confidential
Information which the Recipient can demonstrate:
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                         SCC PROPRIETARY & CONFIDENTIAL
<PAGE>   17
     (i)   is or becomes available to the public through no breach of this or
     any other agreements;

     (ii)  was previously known by the Recipient without any obligation to hold
     it in confidence;

     (iii) is received from a third party free to disclose such information
     without restriction;

     (iv)  is independently developed by the Recipient without the use of
     Confidential Information of the Disclosing Party;

     (v)   is approved for release by written authorization of the Disclosing
     Party but only to the extent of such authorization and without any
     disassembly, reverse engineering, or similar undertaking by Recipient; or

     (vi)  is required by law or regulation to be disclosed, but only to the
     extent and for the purposes of such required disclosure, and only if the
     Recipient first notifies the Disclosing Party of the order and permits the
     Disclosing Party to seek an appropriate protective order.

8.4  OWNERSHIP OF CONFIDENTIAL INFORMATION

     Confidential Information, including permitted copies, shall be deemed the
property of the Disclosing Party. The Recipient shall, within twenty (20) days
of a written request by the Disclosing Party, return all Confidential
Information (or any designated portion thereof) including all copies thereof,
to the Disclosing Party or if so directed by the Disclosing Party, destroy such
Confidential Information. The Recipient shall also, within ten (10) days of a
written request by the Disclosing Party, certify in writing that it has
satisfied its obligations under this section. If the Recipient fails to abide
by its obligations under this section, the Disclosing Party shall be entitled
to immediate injunctive relief in addition to any other rights and remedies
available to it at law or in equity.

                                9.   WARRANTIES

9.1  WARRANTY BY SCC

     9.1.1. SCC warrants and represents to ATGI that any support or other
services that SCC provides to ATGI under this Agreement shall be provided in
accordance with the terms and conditions of this Agreement including that which
is set forth in the Performance Metrics outlined in Exhibit A, which support
and services shall be provided by personnel who are trained and skilled in the
provision of such services and shall be provided in a professional, effective
and efficient manner that equals or exceeds the then-current industry standard
for such services.

     9.1.2. SCC further warrants that any computer software used in connection
with Services, New Services and Add-On Services performed hereunder will: (a)
handle date
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                         SCC PROPRIETARY & CONFIDENTIAL

<PAGE>   18
SCC/ATGI                          Confidential                 Page 16  05/14/99
--------------------------------------------------------------------------------

information before, during and after January 1, 2000, including accepting date
input, providing date output and performing calculations on dates or portions
of dates; (b) function accurately before, during and after January 1, 2000
without change in operations associated with the advent of the new century; (c)
respond to two-digit year/date input in a way that resolves the ambiguity as to
century in a defined and pre-determined manner; (d) store and provide output of
date information in a way that is unambiguous as to century; and (e) recognize
the year 2000 as a leap year.

     9.1.3  SCC has all requisite power and authority and all material
licenses, permits and other authorizations necessary to own an operate its
business and to perform the obligations imposed upon it hereunder and to
provide the Services.

     9.1.4  In the event of any breach of the above warranties, and without
limiting any other rights of ATGI under this Agreement, SCC agrees that it
will, at its sole expense, take appropriate corrective action, including repair
or replacement of the system component not meeting such warranty with respect to
hardware or software and with respect to personnel and business operations,
re-performance of Services or such remedial action as will bring such warranty
into compliance.

9.2  WARRANTY BY ATGI

     9.2.1  ATGI warrants that it has all requisite power and authority and all
material licenses, permits and other authorizations necessary to own and
operate its business and to perform the obligations imposed upon it hereunder.

     9.2.2  ATGI has no knowledge of any existing software viruses contained in
the SCC software or other software used in performing the Services that would
materially impact ATGI's performance hereunder. If ATGI becomes aware of any
such viruses in its software used in connection with its performance hereunder,
it will advise SCC in writing immediately. If SCC's performance hereunder is
materially and negatively impacted by ATGI's failure to notify SCC in a
reasonably timely fashion of its knowledge of such viruses, ATGI shall be
responsible for such reasonable and proximate consequences, and SCC shall be
excused from responsibility hereunder with respect to those matters proximately
affected. Upon learning of such a virus in its software, ATGI shall use its best
efforts to remedy it as soon as possible, at no cost to SCC.

9.3  EXCLUSION

     EXCEPT AS EXPRESSLY PROVIDED HEREIN, SCC MAKES NO EXPRESS OR IMPLIED
WARRANTIES, INCLUDING THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
A PARTICULAR PURPOSE, REGARDING THE SERVICES, NEW SERVICES OR ADD-ON SERVICES
PROVIDED BY SCC HEREUNDER. SCC EXPRESSLY DENIES ANY REPRESENTATION OR WARRANTY
THAT THE SERVICES OR RELATED SYSTEMS SHALL OPERATE UNINTERRUPTED OR ERROR-FREE.
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                         SCC PROPRIETARY & CONFIDENTIAL
<PAGE>   19

                              10. INDEMNIFICATION

10.1     GENERAL INDEMNIFICATION

         SCC shall indemnify and hold harmless ATGI and its corporate
affiliates and the officers, directors, employees, and agents of ATGI and its
corporate affiliates, and the successors and assigns of all of the foregoing,
against and from any and all actual losses, damages, expenses (including,
without limitation, attorneys' fees and costs), claims, suits and liabilities,
whether based in contract or tort (including strict liability), to the extent
that such losses, damages, expenses, demands, claims, suits and liabilities are
legally-imposed, despite the protections that may be available to ATGI and SCC
described in Section 10.3 herein below, and that arise out of or in connection
with: (1) SCC's negligent or intentional acts or omissions, or those of its
employees or agents or subcontractors, (2) the failure of SCC (or its employees
or agents or subcontractors) to fully comply with the terms and conditions of
this Agreement (including but not limited to each SCC representation and each
SCC warranty under this Agreement), or (3) assertions under Worker's
Compensation or similar laws made by persons furnished by SCC. ATGI shall
promptly notify SCC of any written claim, loss or demand for which SCC is
responsible under this Section. If SCC acknowledges in writing the applicability
of the indemnification provisions of this Section 10.1 to a claim or action, SCC
shall have the right to conduct the defense of such claim or action and all
negotiations for settlement or compromise, unless otherwise mutually agreed to
in writing by the Parties hereto. However, ATGI, at its own expense, shall have
the right to participate in the defense of any such suit or proceeding through
counsel of its choosing.

10.2     INTELLECTUAL PROPERTY INDEMNIFICATION

         SCC shall defend, at its sole cost and expense, any claim or action of
any kind against ATGI for alleged violation, infringement or misappropriation of
any patent, copyright, trade secret or other intellectual property right based
on the use of SCC products or services under this Agreement. SCC shall indemnify
and hold harmless ATGI and its officers, directors, employees, and agents and
their successors and assigns against and from any and all actual losses,
liabilities, damages, claims, demands and expenses (including, without
limitation, reasonable attorneys' fees) arising out of or related to any such
claim or action. If SCC acknowledges in writing the applicability of the
indemnification provisions of this Section 10.2 to a claim or action, SCC shall
have the right to conduct the defense of any such claim or action and all
negotiations for settlement or compromise, unless otherwise mutually agreed to
in writing by the Parties hereto. However, ATGI, at its own expense, shall have
the right to participate in the defense of any such suit or proceeding through
counsel of its choosing.

         If any SCC product used to provide the Services under this Agreement
becomes involved in any claim or action described above, or is held to
constitute a violation, infringement or misappropriation of a third party's
intellectual property rights and the use thereof is enjoined, then SCC shall, at
SCC's expense and option:

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                         SCC PROPRIETARY & CONFIDENTIAL

<PAGE>   20

(1) Procure the right to continue using said product so that its use by SCC for
ATGI is lawful;

(2) Modify such product so that its use by SCC for ATGI is lawful (provided
that such modification does not adversely affect the Services provided); or

(3) Replace such product, at no charge to ATGI, with equally suitable,
compatible and functionally equivalent products that lawfully may be used by SCC
for ATGI.

10.3     ATGI IMMUNITY

          To the extent permitted by applicable law, SCC shall be entitled to
not less than the same benefits and protections afforded by any law, regulation,
contract or other applicable rule which extends to ATGI in any form, including
but not limited to governmental or other immunity, indemnification or other
protection which relates to the provision of emergency 9-1-1 services by ATGI
in its capacity as a telecommunications service provider. ATGI acknowledges and
agrees that SCC shall be considered ATGI's authorized agent in this regard.

                          11. LIMITATION OF LIABILITY

11.1     CONSEQUENTIAL DAMAGES

         EXCEPT WITH RESPECT TO SCC'S OBLIGATIONS AS SET FORTH IN SECTIONS 10.1
AND 10.2 TO INDEMNIFY ATGI IN CONNECTION WITH THIRD PARTY CLAIMS AND
INTELLECTUAL PROPERTY INFRINGEMENT, NEITHER PARTY SHALL BE LIABLE TO THE OTHER
FOR ANY INDIRECT, SPECIAL, CONSEQUENTIAL, INCIDENTAL, OR PUNITIVE DAMAGES,
WHETHER BASED UPON LOST GOODWILL, LOST PROFITS, LOSS OF USE OR PERFORMANCE OF
ANY PRODUCTS, SERVICES, OR OTHER PROPERTY, LOSS OR IMPAIRMENT OF DATA OR
SOFTWARE, OR OTHERWISE, AND WHETHER ARISING OUT OF BREACH OF EXPRESS OR IMPLIED
WARRANTY, CONTRACT (INCLUDING THE FURNISHING, PERFORMANCE, OR USE OF ANY
HARDWARE, SOFTWARE OR OTHER PRODUCTS, MATERIALS, OR SERVICES PROVIDED PURSUANT
TO THIS AGREEMENT OR THE PERFORMANCE OR NONPERFORMANCE OF OBLIGATIONS UNDERTAKEN
IN THIS AGREEMENT), TORT, (INCLUDING NEGLIGENCE), STRICT PRODUCT LIABILITY OR
OTHERWISE, REGARDLESS OF WHETHER SUCH PARTY HAS BEEN NOTIFIED OF THE
POSSIBILITY OF SUCH DAMAGES OR IF SUCH DAMAGES COULD HAVE BEEN REASONABLY
FORESEEN.

11.2     MAXIMUM LIABILITY

          TO THE EXTENT NOT OTHERWISE GOVERNED BY FEDERAL, STATE OR LOCAL LAW,
SCC'S ENTIRE LIABILITY FOR ANY CLAIM CONCERNING ITS PERFORMANCE OR
NONPERFORMANCE IN CONNECTION WITH THIS AGREEMENT SHALL BE LIMITED TO AN AMOUNT
EQUAL TO THE [*]

-----------------------
[*]  Information redacted pursuant to a confidential treatment request
     throughout this exhibit.

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                         SCC PROPRIETARY & CONFIDENTIAL

<PAGE>   21

[*]
                                 12. INSURANCE

12.1     INSURANCE COVERAGE

         SCC shall maintain during the term of this Agreement: (1) Workers'
Compensation insurance as prescribed by the law of the state or nation in which
the Work is performed; (2) employer's liability insurance with limits of at
least $500,000 for each occurrence; (3) comprehensive automobile liability
insurance if the use of motor vehicles is required, with limits of at least
$1,000,000 combined single limit for bodily injury and property damage for each
occurrence; (4) Commercial General Liability ("CGL") insurance, including
Blanket Contractual Liability and Broad Form Property Damage, with limits of
at least $1,000,000 combined single limit for bodily injury and property damage
for each occurrence; (5) Professional Liability or Errors and Omissions
insurance in the amount of at least $1,000,000 (one million dollars) for each
occurrence; and excess or umbrella liability at a limit of no less than
$5,000,000 per occurrence and aggregate in excess of the underlying coverage
required above. The CGL, employer liability and automobile liability policies
named above shall designate ATGI and its officers, directors and employees (all
herein referred to in this clause as "ATGI") as an Additional Insured. All such
insurance must be primary and required to respond and pay prior to any other
available coverage.

12.2 EVIDENCE OF COVERAGE

         Upon ATGI's request, SCC shall furnish certificates evidencing the
foregoing insurance. ATGI shall be notified in writing at least thirty (30) days
prior to any cancellation of the policy.

                 13. RECURRING SERVICE ERRORS/LIQUIDATED DAMAGES

13.1 RECURRING SERVICE ERROR

         If any aspect of the Services are deficient on a recurring basis, as
such deficiency is defined herein below as a "Recurring Service Error", then
upon written notice from ATGI to SCC, SCC shall promptly perform a root cause
analysis, in accordance with the escalation procedures contained in this
Agreement and in the SOW, to determine the cause of such recurring service
errors. A Recurring Service Error is any material non-compliance with the
Services which has repeatedly occurred on at least three separate occasions
during any consecutive three month period. Within seven (7) days of receiving
notice from ATGI of a Recurring Service Error, or upon SCC's own discovery of
same, SCC shall provide ATGI with a written copy of its analysis, which shall
include an action plan containing a reasonably detailed description of
corrective action to be taken by SCC and the date by which such corrective
action shall be completed. SCC shall correct such Recurring Service Error within
thirty (30) days of its

--------------------
[*] Information redacted pursuant to a confidential treatment request
    throughout this exhibit.

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                         SCC PROPRIETARY & CONFIDENTIAL

<PAGE>   22

receipt of such notice to the reasonable satisfaction of ATGI in accordance
with its action plan. ATGI may elect to immediately terminate this Agreement
if SCC fails to cure any Recurring Service Error within said thirty (30) day
period.

13.2     LIQUIDATED DAMAGES

         If ATGI elects not to terminate this Agreement as specified herein
above, ATGI may instead elect to compel SCC to pay, or credit, ATGI the
liquidated damages described herein for each Recurring Service Error. Liquidated
damages for each Recurring Service Error shall equal the lesser of: [*] The
Parties acknowledge and agree that all amounts payable to ATGI as "liquidated
damages" under this Article are liquidated damages and not a penalty and are
reasonable and not disproportionate to the presumed damages to ATGI in lost
revenues and other damages that would result from a failure by SCC to comply
strictly with this Agreement. SCC's payment of liquidated damages under this
article shall in no way affect ATGI's right to terminate this Agreement, or any
portion hereof or thereof, or to pursue any other rights except that ATGI's
election of liquidated damages in this regard shall prohibit ATGI from pursuing
any other remedy available to ATGI under this Agreement or applicable law with
respect to recovery of damages related to the particular Recurring Service Error
for which the liquidated damages were paid.

                             14. GENERAL PROVISIONS

14.1     ADVERTISING AND PUBLICITY

         Except for materials already made public, neither Party shall prepare
or distribute any news releases, articles, brochures, speeches, advertisements
or other informational releases concerning this Agreement and the activities
performed hereunder without the prior written consent of the other Party.
Without limiting the generality of the foregoing, neither Party shall publish,
use or disclose the other Party's names or marks (or any variations thereof)
without the prior written consent of the other Party which consent shall not be
unreasonably withheld. ATGI hereby authorizes SCC to utilize ATGI's name as part
of SCC's list of customers in connection with SCC's standard service proposals
and other marketing materials.

14.2     ASSIGNMENT

         This Agreement shall be binding upon the successors and assigns of both
parties, provided, however, that no assignment, delegation or other transfer
shall be made by either party without the prior written approval of the other,
which approval shall not be unreasonably

--------------------
[*] Information redacted pursuant to a confidential treatment request
    throughout this exhibit.

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                         SCC PROPRIETARY & CONFIDENTIAL

<PAGE>   23

withheld. Notwithstanding the foregoing, neither party shall be required to
obtain the approval of the other for any assignment, delegation or transfer of
this Agreement to an affiliate of such party, involving a purchase of all or
substantially all of the assets of such party, or any company with which or into
which such party may merge or consolidate.

14.3     AUTHORITY

         Each Party represents to the other that it has full authority to enter
into and secure performance of this Agreement and that the person signing this
Agreement on behalf of the Party has been properly authorized to enter into this
Agreement. Each Party further acknowledges that it has read this Agreement,
understands it, and agrees to be bound by all of its terms, conditions, and
provisions.

14.4     COMPANY RULES

         SCC's employees and agents shall comply with all of ATGI's security
requirements, rules, and regulations provided to SCC while on ATGI's premises,
and ATGI's employees shall comply with all of SCC's security requirements,
rules, and regulations provided to ATGI while on SCC's premises.

14.5 ESCALATION PROCEDURES

         Any dispute between the Parties under the terms of this Agreement shall
be first submitted to the Project Managers for resolution. It is the
understanding of both Parties that in the event of a dispute that cannot be
resolved by the Project Managers, then it shall be escalated up the levels of
management for each Party pursuant to the procedures set forth in Exhibit D.

14.6 FORCE MAJEURE

         Neither Party shall be liable to the other for any delay or failure to
perform under this Agreement if the delay or failure to perform is without the
fault or negligence of the Party claiming excusable delay and is due to causes
beyond the control of said Party, including, but not limited to acts of God,
war, acts of the government, fires, floods, epidemics, quarantine restrictions,
strikes, labor disputes (including collective bargaining issues), work
stoppages, and freight embargoes.

14.7     GOVERNING LAW

         The validity of this Agreement, the construction and enforcement of
their terms and the interpretation of the rights and duties of the Parties shall
be governed by the laws of the State of Colorado.

14.8     INDEPENDENT CONTRACTORS

         The Parties are performing pursuant to this Agreement only as
independent contractors.

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                         SCC PROPRIETARY & CONFIDENTIAL

<PAGE>   24

Each Party has the sole obligation to supervise, manage, contract, direct,
procure, perform, or cause to be performed its obligations set forth in this
Agreement, except as otherwise provided herein or agreed upon by the Parties.
Except as specified in this Agreement, including Exhibit E, nothing set forth in
this Agreement shall be construed to create the relationship of principal and
agent between SCC and ATGI, and except as expressly provided in Exhibit E, the
Letter Of Agency, neither Party shall act or attempt to act or represent itself,
directly or by implication, as an agent of the other Party or its affiliates or
in any manner assume or create, or attempt to assume or create, any obligations
on behalf of, or in the name of, the other Party unless so instructed by the
other Party in writing or allowed under this Agreement. Nothing herein is
intended or shall be construed to create any partnership or joint venture
relationship between the Parties. Neither a Party nor a Party's subcontractor,
nor the employees of any of them, shall be deemed for any purpose to be
employees of the other Party. Each Party shall be solely responsible for the
withholding and payment of all applicable federal, state, and local personal
income taxes, social security taxes, unemployment and sickness disability
insurance, and other payroll taxes with respect to its own employees.

         SCC acknowledges and agrees that Lucent Technologies and American
Management Systems will act as ATGI's agent in the ordering, provisioning, order
tracking, order status, order management, service activation, and service
fulfillment of the SCC E911 offering. As such, Lucent Technologies and
American Management Systems will be required to contact SCC to obtain technical
assistance and support in connection with this Agreement. To the extent
necessary for Lucent Technologies and American Management Systems to perform
such services, SCC consents to the assignment of ATGI's rights and privileges
under this Agreement.

14.9     JOINT WORK PRODUCT

         This Agreement is the joint work product of representatives of ATGI and
SCC. Accordingly, in the event of ambiguities, no inferences will be drawn or
rules of construction applied against either Party, including the Party that
drafted the Agreement in its final form.

14.10    LAWS, REGULATIONS, PERMITS

         Each Party shall comply, at its own expense, with all applicable
federal, state, county, and local ordinances, regulations, and codes in the
performance of its obligations under this Agreement, including procurement of
required permits and certificates, the Fair Labor Standards Act, and the
Occupational Safety and Health Act for either Party to do business in the U.S.

14.11    NOTICES

         All notices or other communications required or permitted to be given
to a Party under this Agreement (other than communications of a technical
nature, which shall be delivered to such Party's Project Manager) shall be in
writing (unless otherwise specifically provided herein) and delivered or
addressed as follows:

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                         SCC PROPRIETARY & CONFIDENTIAL

<PAGE>   25

If to ATGI:                                  If to SCC:

Advanced TelCom Group, Inc.                  SCC Communications Corp.
Attn: Daniel L. Cruz                         Attn: General Counsel
100 Stony Point Road, Suite 130              6285 Lookout Road
Santa Rosa, CA 95401                         Boulder, Colorado 80301

with a copy to:                              with a copy to:

Advanced TelCom Group, Inc.                  SCC Communications Corp.
Attn: Charlene Curry                         Attn: Chief Financial Officer
100 Stony Point Road, Suite 130              6285 Lookout Road
Santa Rosa, CA 95401                         Boulder, Colorado 80301

All notices or other communications shall be deemed effectively given: when
delivered, if personally delivered; or three (3) days after mailing if mailed
first class or certified or registered mail; or when received by the Party for
which notice is intended if given in any other manner.

14.12    REMEDIES

         The rights and remedies provided herein shall be cumulative and, except
as otherwise limited herein, in addition to any other remedies available at law
or in equity.

14.13    NON-WAIVER

         No course of dealing or failure of either Party to enforce strictly any
term, right, obligation, or provisions of this Agreement or to exercise any
option provided hereunder shall be construed as a waiver of such provision.

14.14    SEVERABILITY

         If any provision of this Agreement shall be held invalid or
unenforceable, such provision shall be deemed deleted from this Agreement and
replaced by a valid and enforceable provision which so far as possible achieves
the Parties' intent in agreeing to the original provision. The remaining
provisions of the Agreement shall continue in full force and effect.

14.15    BINDING EFFECT

         This Agreement shall be binding on, and inure to the benefit of, the
Parties and their respective successors and permitted assigns.

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                         SCC PROPRIETARY & CONFIDENTIAL

<PAGE>   26

                              15.      ENTIRE AGREEMENT

         This Agreement, when fully executed, together with any incorporated
exhibits, constitutes the entire agreement between the parties and supersedes
all promises and representations, if any, whether written or oral, between the
Parties with respect to the subject matter hereof. No modification, amendment,
supplement to, or waiver of this Agreement or any of its provisions, shall be
binding upon the Parties unless made in writing and duly signed by an authorized
representative of each Party.

         IN WITNESS WHEREOF, the Parties hereto have caused this Agreement to be
executed by their duly authorized representatives.

SCC COMMUNICATIONS CORP.                     ADVANCED TELCOM GROUP, INC.

/s/ NANCY K. HAMILTON                        /s/ CURT WHEELING
------------------------------               ----------------------------------
Signature                                    Signature

Nancy K. Hamilton/CFO                        Curt Wheeling, Sr. VP
------------------------------               ----------------------------------
Printed Name and Title                       Printed Name and Title

5/20/99                                      5/15/99
------------------------------               ----------------------------------
Date                                         Date

      SCC
-----------------

  TAR    5/20/99
-----------------
Attorney Approval

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                         SCC PROPRIETARY & CONFIDENTIAL

<PAGE>   27

                                     E9-1-1

                                 CLEARINGHOUSE

                                    SERVICES

                                    EXHIBIT A

                                STATEMENT OF WORK

                                   [SCC LOGO]

                               6285 LOOKOUT ROAD
                      BOULDER, COLORADO, U.S.A. 80301-3343
                                 (303) 581-5600

<PAGE>   28
CONFIDENTIAL                                                           05/14/99

TABLE OF CONTENTS

<TABLE>
<S>                                                                           <C>
INTRODUCTION ..................................................................1

PROJECT INITIATION ............................................................1

       Program Management .....................................................1
       9-1-1 Coordinator ......................................................2
       Service Planning Meetings ..............................................2
       Training ...............................................................2

SERVICE IMPLEMENTATION ........................................................3

       Data Communication .....................................................3
       Data Exchange Requirements .............................................3
       Initial Market Area Information Requirements ...........................3
       MSAG Data Management ...................................................4
       Methods & Procedures ...................................................4

DAILY OPERATIONS ..............................................................4

       Subscriber Record Updates (Service Order Input "SOI" File) .............4
       Error Processing .......................................................5
       Post-Processing Acknowledgment .........................................5
       Record Transactions and Handling .......................................6
       Data Processing Intervals ..............................................7
       Data Exchange ..........................................................7
       New Client Markets .....................................................7

MEASURING PERFORMANCE .........................................................8

       Performance Metrics ....................................................8
       Primary Metric 1 - Elapsed Time to Post Service Orders .................9
       Primary Metric 2 - Inbound Correctable Data Error Rates ................9
       Primary Metric 3 - Error Resolution by Class of Error ..................9
       Primary Metric 4 - Error Resolution Intervals .........................10
       Quarterly Alliance Performance Reviews ................................10
       Audits ................................................................11

CUSTOMER SUPPORT .............................................................12

       Introduction ..........................................................12
       Hours of Operation ....................................................12
       Overview of Support Plan Activities ...................................12
       Support Contact Procedures ............................................13
       Support Contact List ..................................................14
       Error Severity Levels & Correction Procedures .........................14
       SEVERITY LEVEL 1 ......................................................14
       SEVERITY LEVEL 2 ......................................................15
       Status & Reports ......................................................16
       Notification and Escalation Procedures ................................16

RESPONSIBILITIES OF THE PARTIES ..............................................17
</TABLE>

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SCC - ATGI STATEMENT OF WORK                                                  ii
<PAGE>   29

CONFIDENTIAL                                                           05/14/99

INTRODUCTION

          This Statement of Work, executed by SCC Communications Corp, is for
          delivery of E9-1-1 data management services.

          SCC will be Client's agent and single point of contact for E9-1-1
          database management and for developing and maintaining the
          relationships necessary to provide the Client's customers optimum
          access to the 9-1-1 infrastructure. SCC will receive Service Order
          Input (SOI) records and validate them against the Master Street
          Address Guide (MSAG), correct records which are not MSAG valid, and
          distribute the SOI records for updates to the appropriate ALI Provider
          (such as an RBOC or other incumbent Independent Telephone Company).

PROJECT INITIATION

     PROGRAM MANAGEMENT

          SCC has designated a Program Manager to act as the primary interface
          between the two companies. Likewise, the Client will also designate a
          Program Manager to act as the primary interface between the two
          companies. The Program Managers shall be responsible for ensuring the
          successful achievement of project milestones as well as communicating
          project status within their respective organizations.

          The Program Managers shall work together to create an Implementation
          Plan within 30 days of contract signing. This Implementation Plan
          shall take into account key milestone dates to be achieved in the
          project. SCC will have responsibility for maintaining the
          Implementation Plan.

          SCC and the Client shall meet on a periodic basis either in person or
          through teleconferencing. Frequency of meetings shall be mutually
          agreed upon by the Program Managers. Each Party shall provide status
          on the work being performed in support of the Program. Mutually, the
          companies may elect to forego all or some of such meetings in lieu of
          periodic status reports.

          The Program Managers will maintain and distribute a list of key SCC
          and Client contacts (technical, operational, and managerial).

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SCC - ATGI STATEMENT OF WORK                                                   1

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CONFIDENTIAL                                                           05/14/99

     9-1-1 COORDINATOR

          Client will appoint a 9-1-1 Coordinator who will serve as the primary
          SCC interface for "daily operations". This person is responsible for
          working with SCC in reporting/verifying problems, reviewing/rectifying
          error reports and performing system administrative duties (on Client
          side) such as data backups, archives, etc. This person will have
          primary responsibility for understanding SCC/Client 9-1-1 data
          processing, Methods and Procedures, and will facilitate ongoing
          communications with SCC (e.g. Quarterly Reviews).

     SERVICE PLANNING MEETINGS

          During the early stages of the project, the Program Managers will work
          together to determine the appropriate planning, design, and
          requirements definition meetings to be held to ensure successful
          delivery of E9-1-1 data management services.

          These planning sessions will be held with the Client's appropriate
          technical and operational groups to ensure a solid understanding of
          SCC data exchange procedures/requirements and daily operations.

     TRAINING

          SCC will provide up to two (2) days of 9-1-1 training concurrently to
          up to ten (10) designated Client 9-1-1 Coordinator(s) for working with
          SCC (data exchange procedures, data processing procedures, new market
          entry procedures, escalation procedures, etc.) Training will be
          conducted at SCC's headquarters in Boulder, Colorado.

          Additional training requested by Client (e.g. training for remote
          facilities) will be provided by SCC on a time and materials basis. SCC
          shall provide estimated costs in advance of incurring any expenses on
          Client's behalf and subject to Client's approval.

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SCC - ATGI STATEMENT OF WORK                                                   2
<PAGE>   31

CONFIDENTIAL                                                           05/14/99

SERVICE IMPLEMENTATION

     DATA COMMUNICATION

          SCC will work with Client to determine the most appropriate method of
          electronic data communication. Depending on subscriber record volume,
          either Dedicated Circuit or Dial-Up connectivity may be selected.

          SCC and Client will establish a Primary and Secondary (Back-up)
          method of data communication. Typically, the Secondary method of data
          exchange will be Dial-Up connectivity (if Primary is Dedicated
          Circuit) or FAX (if Primary is Dial-Up). Data Communications
          requirements are detailed in SCC's "Data Exchange Guidelines"
          document.

          Client will bear data communications facilities costs from Client
          location to SCC. SCC will be responsible for identifying, and Client
          will be responsible for ordering, any required circuits and ensuring
          that the Program Managers are aware of any circuit installation and
          testing time requirements.

     DATA EXCHANGE REQUIREMENTS

          Data exchange requirements (Input and Output files with required data
          elements) are provided in SCC's "Data Exchange Guidelines" document
          detailing file specifications, service order process, data exchange
          requirements and file naming conventions.

     INITIAL MARKET AREA INFORMATION REQUIREMENTS

          Prior to initiating service in any market area, the Client agrees to
          provide SCC with the following:

          o    Market Areas (counties/communities served; required for MSAG
               validation)

          o    Approximate number of TNs in each Market Area

          o    All assigned NPA/NXX's and their associated Communities

          o    Letter of Agency that allows SCC to act on behalf of the Client
               to acquire MSAGs from host E9-1-1 database providers

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SCC - ATGI STATEMENT OF WORK                                                   3

<PAGE>   32

CONFIDENTIAL                                                           05/14/99

     MSAG DATA MANAGEMENT

          SCC will work with the appropriate ALI Provider to obtain current
          MSAGs for all service areas identified by the Client. SCC utilizes
          these MSAGs to validate subscriber addresses on Client SOI data
          records as "MSAG correct" before transmitting E9-1-1 database updates
          to the appropriate ALI provider. SCC will refresh and update these
          MSAGs on a quarterly basis and will provide MSAGs in a standardized
          electronic data format conforming to "SCC's Guidelines for Data
          Exchange."

     METHODS & PROCEDURES

          To ensure a high and consistent level of service, SCC has developed
          Methods and Procedures to assist Client and SCC when handling specific
          9-1-1 data management activities. Methods and Procedures are developed
          as operational guidelines that identify the tasks, dependencies, and
          the responsibilities of all parties for the completion of specific
          activities.

          These Methods and Procedures will be reviewed with the Client's
          operations group during the early stages of the program.

          o    New Market Area Testing

          o    ANI/ALI Error Reporting & Resolution

          o    After Hours Problem Reporting

          o    Escalation Procedures

          o    Record Processing

          o    NPA Splits for ALECs

          o    Local Number Portability Data Processing

DAILY OPERATIONS

     SUBSCRIBER RECORD UPDATES (SERVICE ORDER INPUT "SOI" FILE)

          The largest volume of transactions processed by the NDSC fall into the
          category of subscriber record updates. These transactions are
          generated as a result of telephone customer requests for new telephone
          service or a change in their current service. Service Order Input
          "SOI" files are generated and transmitted to SCC's NDSC in NENA II
          format.

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SCC - ATGI STATEMENT OF WORK                                                   4

<PAGE>   33

CONFIDENTIAL                                                           05/14/99

          As SOI files are received by the NDSC, SOI records are processed on
          the Transaction Services System (TSS) and matched against the E9-1-1
          reference databases (e.g. translation tables, MSAG, prefix tables,
          security tables, etc.). Valid SOI data is re-formatted into the
          appropriate format for distribution to the appropriate ALI Provider.

     ERROR PROCESSING

          SOI data not matching the specified and standardized parameters for
          valid E9-1-1 data fall out into a controlled, fully traceable error
          database in which errors are grouped based on date, type and area.

          SCC's standard practice is to correct only MSAG errors, i.e. "701"
          errors (House number out of range in MSAG) and "709" errors (Street
          not found in MSAG). These are the only errors for which SCC can access
          the appropriate data sources for accurate correction. These
          corrections are usually completed the same day of error notification.

          The Client will make every effort to supply error-free data to SCC.
          SCC will document the SCC action taken to correct MSAG errors in an
          ANNOTATED ERROR FILE (see "Data Exchange Guidelines" document).
          Annotated Error files are returned to Client within 24 hours of error
          detection (48 hours after SCC's receipt of SOI file). Client will be
          responsible for working with its end-user customers in resolving these
          errors. If there is a delay in the creation of the Annotated Error
          File, SCC will notify Client's 9-1-1 Coordinator.

     POST-PROCESSING ACKNOWLEDGMENT

          Once subscriber record update processing is complete, SCC returns
          Confirmation(s) and Statistics file(s) to the Client. The STATISTICS
          FILE contains the number of records received, the number of records
          processed without error, the number of records processed with errors,
          and the number of errors by type. The CONFIRMATIONS FILE contains a
          copy of each record sent to SCC as input (SOI file) in NENA II format.
          The Confirmations file contains an error/status field noting status of
          9-1-1 data processing.

          Records sent to ALI Providers are sent in a single batch file to each
          Provider. SCC routes these records to the appropriate recipient based
          on the following categories:

          o    SCC NDSC TSS System for Ameritech

          o    SCC NDSC TSS System for US West

          o    SCC NDSC TSS System for BellSouth

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SCC - ATGI STATEMENT OF WORK                                                   5

<PAGE>   34

CONFIDENTIAL                                                           05/14/99

          o    Other Non-SCC ALI Provider systems (e.g. Pacific Bell, Bell
               Atlantic, GTE, NYNEX, Sprint, Southwestern Bell, Cincinnati Bell,
               Rochester Telephone, SNET)

          Records for each TSS system are processed separately after being
          parsed and routed from the original batch file. Thus, SCC returns a
          Statistics file and a Confirmations file from each TSS system. These
          Statistics and Confirmation files are traceable to the original batch
          file by the matching sequence number contained in the file names.

          The Confirmation files returned by SCC contain the NENA II record
          which includes a status/error field. If errors occurred while
          validating the records against the MSAG, SCC populates the
          status/error field of the NENA II record along with the appropriate
          status/error code (see SCC's "Data Exchange Guidelines" document).

          When no error occurs, the status/error field will contain a "000"
          (triple zero-confirmation that the records were successfully entered
          into host ALI database) for records processed on NDSC E9-1-1
          databases, and a "FWD" code (indicating the record validated against
          the MSAG and was successfully transmitted to the ALI Provider) for
          records transmitted to Non-NDSC E9-1-1 database providers.

     RECORD TRANSACTIONS AND HANDLING

          SCC will work with Client to determine the most appropriate method of
          electronic data communication. Depending on subscriber record volume,
          either Dedicated Circuit or Dial-Up connectivity may be selected.

          Under normal circumstances, error free records received electronically
          by 2:30pm MST, Monday through Friday are prepared and sent the same
          day to the host ALI provider using the standard data transmission
          procedures.

          THE FOLLOWING IS AN EXAMPLE OF A TRANSACTION:

          o    SCC receives SOI Records via a batch file in NENA II format

          o    Records are processed on SCC's Transaction Services System and
               validated against the appropriate MSAG.

          o    SCC produces a Statistics File and a Confirmation File with a
               "one to one" confirmation for every record sent in SOI file
               (status codes = "000", "FWD", or Error Code).

          o    Records with MSAG-related ("701" & "709") errors are time stamped
               and placed into the Error Queue for analysis and correction by
               SCC Data Analyst Unit.

          o    SCC resolves MSAG-related ("701" & "709") errors if possible.

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SCC - ATGI STATEMENT OF WORK                                                   6

<PAGE>   35

CONFIDENTIAL                                                            05/14/99

          o    SCC produces an Annotated Error File detailing the action taken
               by SCC to correct MSAG-related errors.

          o    MSAG valid records are re-formatted per ALI Provider requirements
               and transmitted to ALI Provider per ALI Provider data processing
               windows.

          o    SCC archives daily Confirmation and Statistics Files.

     DATA PROCESSING INTERVALS

          With respect to non-facsimile, i.e., electronic file format;
          error-free transactions are processed within 24 hours of receipt. In
          regions where SCC maintains ALI records, error-free records are sent
          through SCC's TSS and updated in ALI within 24 hours, and for all
          other regions, SCC processes records against the appropriate MSAG in
          the TSS and prepares error-free records for transmission to the proper
          host LEC. In rare cases, routine processing may be suspended for
          special operations. These situations are coordinated in advance when
          possible. In any situation where data processing may be interrupted
          (e.g. Systems Maintenance, NPA Splits or Overlays), the Client's
          assigned 9-1-1 Coordinator will be notified. When known, SCC will
          provide Client with 72 hours advance notice.

     DATA EXCHANGE

          Except as otherwise provided herein regarding facsimile exchange of
          data during 1998, data (Input and Output files) will be exchanged per
          SCC's "Data Exchange Guidelines" document.

     NEW CLIENT MARKETS

          The Client shall provide SCC with a 45-day written notice for all
          prospective market entries. Client will also provide SCC with
          information as outlined in the "Initial Market Area Information
          Requirements" section of this Statement of Work.

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SCC - ATGI STATEMENT OF WORK                                                   7

<PAGE>   36

CONFIDENTIAL                                                            05/14/99

MEASURING PERFORMANCE

     PERFORMANCE METRICS

          SCC applies a set of standard performance metrics to measure the
          E9-1-1 data management services provided to Client. Performance
          Metrics are provided to Client once a minimum level of 5000 subscriber
          records are under SCC management and are being exchanged in electronic
          file format. SCC requires a 90-day period after initiation of data
          management services to establish the first "baseline" set of Metrics.
          Metrics reports are supplied to Client on a monthly basis.

          The Primary Performance Metrics are:

          o    Elapsed Time to Post Service Order Updates

          o    Inbound Correctable Data Error Rates

          o    Error Resolution Interval by Class of Error

          o    Client Error Resolution Intervals

          The Primary Metrics are reported on a variety of criteria. Listed
          below are the Primary Metrics and the breakdowns used for each metric.
          Additionally, measurable performance standards are shown. Measurable
          standards for service order processing, error resolution intervals and
          error frequencies by error type define benchmarks against which SCC is
          assessed.

<TABLE>
<CAPTION>
                                PRIMARY PERFORMANCE METRICS
          -------------------------------------------------------------------------
                     METRIC TYPE                 STATISTICAL BREAKDOWNS POSSIBLE
          --------------------------------     ------------------------------------
                                               REGION   STATE   COMPANY   NPA   NXX
                                               ------   -----   -------   ---   ---
          <S>                                  <C>      <C>     <C>       <C>   <C>
          1. Elapsed Time to Post Service
             Orders                              Yes     Yes      Yes     Yes   Yes

          2. Inbound Correctable Data Error
             Rates                               Yes     Yes      Yes     Yes   Yes

          3. Error Resolution Interval by
             Class                               Yes     Yes      Yes     Yes   Yes

          4. Client Error Resolution
             Intervals                           Yes     Yes      Yes     Yes   Yes
</TABLE>

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SCC - ATGI STATEMENT OF WORK                                                   8

<PAGE>   37

CONFIDENTIAL                                                            05/14/99

     PRIMARY METRIC I - ELAPSED TIME TO POST SERVICE ORDERS

          Definition:

          This metric is the elapsed time from the initial receipt of a valid
          service order until that order is processed on TSS and is transmitted
          to the appropriate ALI Provider.

          Performance Standard:

          The elapsed time to post SOI updates is less than twenty-four (24)
          hours.

          Direct Measurement of Quality

          98% of all service orders received are processed on a SCC NDSC E9-1-1
          database or are transmitted to the appropriate ALI Provider within 24
          hours of receipt.

     PRIMARY METRIC 2 - INBOUND CORRECTABLE DATA ERROR RATES

          Definition:

          The number of correctable data errors received compared to the total
          number of records received.

          Performance Standard:

          The routine reports are broken down by source state. SCC retains data
          records to support error analysis by telephone service provider, NPA
          NXX, and entity. These reports are run monthly to support error
          reduction efforts.

          Direct Measurement of Quality

          Inbound correctable error rates will not exceed 10% of the total
          service orders processed for a given time period.

     PRIMARY METRIC 3 - ERROR RESOLUTION BY CLASS OF ERROR

          Definition:

          This metric is defined as the elapsed time from when SCC receives an
          errant service order until the order is corrected and processed on the
          Transaction Services System.

          Performance Standard:

          SCC reports the resolution interval for the six most common error
          types. Since both parties contribute to the error resolution interval,
          each party commits to an on-going effort to shorten the time
          intervals.

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SCC - ATGI STATEMENT OF WORK                                                   9
<PAGE>   38

CONFIDENTIAL                                                            05/14/99

          Direct Measurement of Quality

          The number of unresolved errors in any month are less than 0.2% of the
          total number of TNs in the database at the end of the month. Errors
          that cannot be resolved and are referred back to the Client for
          resolution are not included in this metric.

     PRIMARY METRIC 4 - ERROR RESOLUTION INTERVALS

          Definition:

          This metric is defined as the elapsed time from:

          a) When the Customer is notified of the existence of an uncorrectable
             error condition;

          b) Through the time when the Customer notifies SCC of the correct
             resolution to the problem.

          Performance Standard:

          Notification by SCC occurs within an average of 12 hours after
          identifying a problem requiring the Client's intervention. The
          Performance Standard for resolution of the problem shall be mutually
          agreed upon.

     QUARTERLY ALLIANCE PERFORMANCE REVIEWS

          While the Performance Metrics are reported to the appropriate
          personnel with a predefined frequency, the two companies will meet on
          a quarterly basis to formally review the performance of the alliance,
          end user customer feedback, upcoming major events, important industry
          trends, new government regulations, and the status of new product
          introduction plans. SCC will also present Client with process
          improvement recommendations intended to lower data errors and improve
          data quality.

          These review meetings shall alternate between SCC and the Client
          facilities (unless mutually agreed otherwise) and be attended by the
          appropriate operating management group of each company. The host
          company is responsible for the preparation of the agreed upon agenda.

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SCC - ATGI STATEMENT OF WORK                                                  10

<PAGE>   39

CONFIDENTIAL                                                            05/14/99

     AUDITS

          SCC maintains a copy of all TN records submitted by the service order
          process including an audit trail for every transaction record. SCC
          will perform annual reconciliations of Client source data with ALI
          data for data residing on SCC's systems (NDSC regions). For non-NDSC
          regions, SCC will perform annual reconciliations or audits when Client
          has received the commitment and timeframe from the non-NDSC LEC to
          deliver to SCC an ALI extract file of Client data. Client will be
          responsible for procurement costs for acquiring the extract file from
          non-NDSC LECs.

          As part of SCC's standard Clearinghouse Services, SCC performs a
          detailed reconciliation of 50% of all Client data each year (based on
          number of TNs under SCC management). Reconciliations are staged
          throughout the year (approximately 12.5% each quarter); all data to
          complete the audit cycle every two (2) years. Problematic errors
          identified by either SCC or Client may be subject to more frequent
          reconciliations based on mutual agreement to do so. SCC understands
          the importance of the reconciliation process and the benefits to the
          integrity of the 9-1-1 data that is derived and will work with the
          Client in the prioritization and scheduling of reconciliations.

          SCC will provide Client with copies of reconciliation reports at the
          completion of regional reconciliations. Reconciliation reports will
          contain the number of records processed, number of discrepancies by
          type, discrepancy resolution status by record (either corrected or
          referred to Client).

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SCC - ATGI STATEMENT OF WORK                                                  11

<PAGE>   40

CONFIDENTIAL                                                            05/14/99

CUSTOMER SUPPORT

     INTRODUCTION

          This support plan represents SCC's commitment to the ongoing support
          of the E9-1-1 Services being provided to Client and Client's
          customers. The following pages provide useful and important
          information for effective and efficient data management services
          support. This support plan includes:

               o an overview of SCC support plan activities

               o support contact procedures

               o assigned SCC points of contact

               o definitions of errors by severity level and correction
                 procedures

               o status reporting, and

               o notification and escalation procedures

     HOURS OF OPERATION

          SCC analysts provide data management services on a 12 hour (6:00 am to
          6:00 pm MST) by 5 day (Monday through Friday) basis. For emergency
          situations occurring outside of the 12x5 work hours, SCC will provide
          24 hour, seven days per week support services. The computer operations
          support staff are available at the NDSC facility around-the-clock for
          emergency services. NDSC management teams are also available via a
          nationwide radio pager network and supplemental staff are available in
          the event specialized services are required to respond to specific
          situations.

     OVERVIEW OF SUPPORT PLAN ACTIVITIES

          When 9-1-1 data problems arise, SCC provides prompt resolution
          assistance. For day to day data management and processing matters, the
          Client will always contact their designated SCC data analyst for
          problem resolution, communication and operational matters.

          The SCC data analyst is also the first point of contact for system or
          data communication problems that may be impacting the Client's ability
          to process service orders that occur within the normal workday. After
          business hours, critical problems as defined in the severity level
          descriptions below shall be reported to the NDSC Computer Operation's
          operator. The on-duty operator will assess the priority of the call,
          and when appropriate will page SCC personnel to request assistance for
          resolving an urgent problem.

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SCC - ATGI STATEMENT OF WORK                                                  12

<PAGE>   41

CONFIDENTIAL                                                            05/14/99

          If a problem is determined to be the responsibility of Client or
          Client's customer (Client network, PSAP network, CPE, etc.), SCC
          will provide any available information to assist in the resolution of
          the problem. SCC expects that Client will have the same resolution
          time commitment for Severity Level 1 and 2 problems as are stated in
          this support plan.

          In the event the Client experiences a critical problem that either
          prohibits the transfer of service orders, or requires a backup or
          alternate processing method to be invoked, SCC should be notified,
          following the support contact procedures outlined below.

     SUPPORT CONTACT PROCEDURES

          Support is available 24 hours/day, 7 days/week, for reporting network
          problems, data communication issues or emergency ALI queries. Client
          will designate a primary contact person for working with SCC
          (generally the 9-1-1 Coordinator) personnel on support matters.

          SCC will log the call, problem description, and other pertinent
          information. SCC's response will coincide with the severity level of
          the call. Every attempt will be made to answer questions and provide
          resolution when the call is received, however, call back situations
          may occur particularly after normal business hours.

                    Examples of emergency situations that are supported by
                    computer operations staff, for NDSC provisioned systems, on
                    a 24-hour basis include:

                      o Network hardware, circuit, or ALI link failure

                      o ALI node failure on both sides of the PSAP

                      o Emergency ALI queries during dual ALI node failure

                    Computer operations staff can complete ALI lookup requests
                    for the NDSC provisioned ALI databases. ALI queries are
                    handled after hours on an emergency and exception basis and
                    in accordance to the operational procedures established by
                    the ALI host provider.

          On non-NDSC systems, computer operators can conduct TSS queries on an
          exception and emergency basis. SCC recognizes that the need of
          providing this service is infrequent and whenever possible, the ALI
          database is the absolute preferred source for such inquires. SCC will
          provide our customers with the methods and procedures for requesting
          emergency ALI or TSS queries.

          SCC only provides the emergency ALI or TSS lookup services to SCC
          clients. PSAPs will not be given access to this service.

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SCC - ATGI STATEMENT OF WORK                                                  13

<PAGE>   42

CONFIDENTIAL                                                            05/14/99

     SUPPORT CONTACT LIST

<TABLE>
<CAPTION>
                POINT OF CONTACT                            TELEPHONE NUMBER
       ---------------------------------                   ------------------
       <S>                                                 <C>
       Business Hours

       Data Analyst:                                         (303)    -
                    --------------------                          ---- ----
       Data Analyst:                                         (303)    -
                    --------------------                          ---- ----

       After Hours, Holidays and Weekends

       NDSC Computer Operations                              (303) 581-5782
</TABLE>

     ERROR SEVERITY LEVELS & CORRECTION PROCEDURES

          SCC will correct errors and malfunctions in the service(s) that cause
          such service(s) either to be unavailable for use by Client or that
          materially fails to conform to the applicable specifications for such
          service(s) as described in the Client services agreement.

          Client may report errors to SCC either verbally or by written notice.
          Likewise, SCC may report errors detected by SCC personnel to Client
          either verbally or by written notice. If notice of the error is
          initially made verbally, the reporting party shall provide written
          notice (facsimile or electronic mail) within 24 hours of the initial
          verbal notification. Client shall use reasonable care to verify
          whether an error exists in, or is a result from, the service(s) prior
          to reporting such to SCC.

          The service(s) error correction procedures to be followed are
          dependent on the severity of the error as defined below. Each error
          severity level defines the actions that will be taken by SCC for
          Response Time, Resolution Time and Resolution Procedure.

     SEVERITY LEVEL 1

          DEFINITION: The E9-1-1 Database Services are severely and critically
          impaired, where major functions are completely inoperative and those
          major functions are critical to operation of the E9-1-1 Database
          Services.

          RESPONSE TIME: SCC will acknowledge the issue with a telephone
          conference call with Client within 30 minutes of Client's initial
          notification to SCC. SCC will notify Client of any service affecting
          errors within 30 minutes of the discovery of such errors

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SCC - ATGI STATEMENT OF WORK                                                  14

<PAGE>   43

CONFIDENTIAL                                                            05/14/99

          RESOLUTION TIME: SCC will apply all reasonable efforts to provide a
          resolution within 6 hours of the notification from Client. If
          resolution cannot be provided within 6 hours, SCC will initiate the
          internal escalation procedure within the 6-hour period to ensure
          resources are appropriately assigned for problem resolution efforts.

          RESOLUTION: SCC will correct the service or provide a procedure for
          Client to bypass or work around the error condition in order to
          continue operations. If a bypass procedure is utilized, SCC will
          provide Client an acceptable action plan for the development of the
          final error correction activity and SCC will continue error resolution
          activity until full service is restored to Client.

                    Examples of Severity Level 1 conditions:

                      o Critical network or data communications error on an SCC
                        system that prevents us from receiving service order
                        files.

                      o TSS system failure that prohibits the processing of
                        service order files within the contractually defined
                        response times.

     SEVERITY LEVEL 2

          DEFINITION: The E9-1-1 Database Services are impaired and some
          functions are not operating, but those functions are not mandatory or
          critical to the operation of the E911 Database Services.

          RESPONSE TIME: SCC will respond with a telephone conference call with
          Client within 6 business hours of Client's initial notification of
          problem or error.

          RESOLUTION TIME: SCC will provide resolution within 14 days of the
          notification.

                    Examples of Severity Level 2 conditions:

                      o Unexpected character in a returned confirmation record

                      o Inability to post return reports or files for electronic
                        pick up by the customer.

                    RESOLUTION: SCC will correct the service or provide Client
                    with a procedure to bypass or work around the error
                    condition in order to continue operations. If a bypass
                    procedure is utilized, SCC will provide Client an acceptable
                    action plan for the development of the final error
                    resolution and SCC will continue error resolution activity
                    until full service is restored to Client.

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SCC - ATGI STATEMENT OF WORK                                                  15

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CONFIDENTIAL                                                            05/14/99

     STATUS & REPORTS

          SCC will provide verbal correction status reports on Severity Level 1
          problems at intervals of not less than 4 hours to a designated Client
          representative. SCC shall provide verbal correction status reports on
          Severity Level 2 errors at intervals of not less than once per week to
          a designated Client representative.

          Client agrees to provide SCC, at the time of notification, all
          pertinent data requested by SCC to properly analyze an error
          condition. If SCC is unable to resolve the problem within the
          allotted resolution time due to a lack of data required to analyze the
          error condition, the internal Escalation Procedures defined below will
          be initiated. Client agrees to provide timely assistance if SCC
          requests additional information and data concerning the error
          condition.

          Each month, Client will provide SCC, not later than the 15th day of
          the relevant month, a list of Client's operating companies which
          report shall detail every NPA/NXX (area code and prefix) associated
          with each such company. Upon its receipt of such report in each such
          month, SCC will provide Client with a report reflecting the 9-1-1
          record counts (access lines), breaking out the NPA/NXX's for each
          Client operating company. SCC shall attach its report to the monthly
          invoice rendered by SCC pursuant to Exhibit B.

     NOTIFICATION AND ESCALATION PROCEDURES

          SCC will provide Client with a notification document that includes a
          listing of key SCC employees by function, telephone numbers and
          24-hour pager numbers. This information is provided to Client in case
          an extraordinary situation develops that requires immediate
          notification. The Client will provide SCC with a similar list of
          contact numbers for emergency situations. It is the responsibility of
          each party to update and publish these lists on a regular basis.

          In conjunction with the notification process, SCC will provide for
          escalation processes within the standard guidelines set forth in the
          Clearinghouse Services Methods and Procedures.

--------------------------------------------------------------------------------
SCC - ATGI STATEMENT OF WORK                                                  16

<PAGE>   45

CONFIDENTIAL                                                            05/14/99

RESPONSIBILITIES OF THE PARTIES

<TABLE>
          <S>                                                         <C>
          TRANSPORT - NETWORK PLANNING
              Data Comm from Client to SCC ...........................SCC/Client
              Data Comm from SCC to ALI Provider .....................SCC/ALI Provider
              Liaison to SCC .........................................Client

          TRANSPORT - NETWORK SUPPORT
              Data Comm from Client to SCC ...........................Client
              Data Comm from SCC to ALI Provider .....................SCC/ALI Provider
              Problem Investigation ..................................SCC/ALI Provider/Client

          DATABASE SERVICES
              Provide Initial Subscriber Records .....................Client
              Provide Ongoing Updates to Subscriber Information ......Client
              Address Issues/MSAG ....................................SCC/ALI Provider
              Initial Database Creation ..............................SCC/ALI Provider
              Data Integrity .........................................SCC
              Database Maintenance ...................................SCC/ALI Provider/Client
              Error Correction .......................................SCC/ALI Provider/Client
              "No Record Found" Misroute Investigation ...............SCC/ALI Provider/Client
              Data Discrepancy Investigation .........................SCC
              Database Reconciliation ................................SCC

          OPERATIONS
              System/Application Security ............................SCC
              Computer Operations ....................................SCC
              Support Data Center Issues .............................SCC
              Perform Emergency Database Investigations ..............SCC/Client/ALI Provider

         ADMINISTRATION
              Public Relations .......................................Client/SCC
              Public Service Commission Interface ....................Client/SCC
              Subscriber Billing .....................................Client
</TABLE>

--------------------------------------------------------------------------------
SCC - ATGI STATEMENT OF WORK                                                  17

<PAGE>   46

                                     E9-1-1

                                  CLEARINGHOUSE

                                    SERVICES

                                   EXHIBIT B

                               PRICING & PAYMENT

                                     TERMS

                                   [SCC LOGO]

                               6285 Lookout Road
                      Boulder, Colorado, U.S.A. 80301-3343
                                 (303) 581-5600

<PAGE>   47

ATGI CONFIDENTIAL

Exhibit B - Fees and Payment
--------------------------------------------------------------------------------
                                   EXHIBIT B

                           FEES AND PAYMENT SCHEDULE

Customer will pay SCC the following fees in the amounts and in the manner
outlined below:

I. NON-RECURRING ENGINEERING FEE (NRE)

     A [*] NRE, if not already paid by Customer, is due within thirty (30) days
after the Agreement is fully executed by the parties.

     This NRE is a one time charge to initiate the work related to engineering
and implementation of Customer's Clearinghouse Services E9-1-1 solution
described in Exhibit A. This NRE will not be charged on a per market basis for
each market "roll-out." This NRE is non-refundable and payment is not dependent
upon performance of the work associated therewith. The work items covered by
this NRE are fully described in Exhibit A but include the following:

o MSAG data acquisition and build out

o Network Configuration, development of data exchange interfaces to Customer,
  hardware preparation and installation

o Primary and Secondary data exchange connectivity configurations

o File transfer testing and integration testing

o System configuration to support Local Number Portability processing

II. MONTHLY BASE SERVICE FEE (MBF) PER BASE RECORD PER MONTH

<TABLE>
<CAPTION>
Total Number of Actual Base Records                        Fee
-----------------------------------          -------------------------------
<S>                                          <C>
       [*]                                   [*]
       [*]                                   [*]
       [*]                                   [*]
       [*]                                   [*]
       [*]                                   [*]
       [*]                                   [*]
</TABLE>

     This ongoing MBF covers the costs of maintaining Customer's Base Records.
MBFs are invoiced in arrears on a monthly, per Base Record basis, on or about
the 1st day of each month. Payment of such invoices are due thirty (30) days
from the date stated on the relevant invoice. MBFs will begin to accrue in the
first month in which Customer's E911 data is converted to SCC and SCC begins
accepting and managing live E911 data on behalf of Customer; however, if such
date occurs after the tenth (10th) day of such first month, MBFs will begin to
accrue as of the next month.

--------------------
[*] Information redacted pursuant to a confidential treatment request
    throughout this exhibit.

--------------------------------------------------------------------------------
CUSTOMER - EXHIBIT B                PAGE B-1                             5/14/99
                                  CONFIDENTIAL
<PAGE>   48

                                     E9-1-1

                                  CLEARINGHOUSE

                                    SERVICES

                                   EXHIBIT C

                               MODIFICATION ORDER

                                   [SCC LOGO]

                               6285 Lookout Road
                      Boulder, Colorado, U.S.A. 80301-3343
                                 (303) 581-5600

<PAGE>   49

ATGI                                                                CONFIDENTIAL

                                                  EXHIBIT C - MODIFICATION ORDER
--------------------------------------------------------------------------------
                         EXHIBIT C - MODIFICATION ORDER

NEW SERVICES - PRODUCT NAME:

-----------------------------------------

PRODUCT DESCRIPTION:

--------------------------------------------------------------------------------

--------------------------------------------------------------------------------

--------------------------------------------------------------------------------

IMPLEMENTATION PLAN (Attached to this Add-On Order)

ANTICIPATED DATE OF SERVICE START:

--------------------------------------------------------------------------------

NON-RECURRING CHARGES - DESCRIPTION AND AMOUNT:

--------------------------------------------------------------------------------

--------------------------------------------------------------------------------

RECURRING CHARGES AND EFFECTIVE DATE:

--------------------------------------------------------------------------------

This Add-On Order shall be included as Addendum # _____ to the 9-1-1 Services
Agreement.

CUSTOMER, INC.                               SCC COMMUNICATIONS CORP.

---------------------------------            -----------------------------------
Signature                                    Signature

---------------------------------            -----------------------------------
Name/Title                                   Name/Title

---------------------------------            -----------------------------------
Date                                         Date

--------------------------------------------------------------------------------
EXHIBIT C                           PAGE C-1                             5/14/99
<PAGE>   50

                                     E9-1-1

                                  CLEARINGHOUSE

                                    SERVICES

                                   EXHIBIT D

                              ESCALATION PROCEDURES

                                   [SCC LOGO]

                               6285 Lookout Road
                      Boulder, Colorado, U.S.A. 80301-3343
                                 (303) 581-5600

<PAGE>   51

ATGI                                                                         SCC
PROPRIETARY - USE PURSUANT TO INSTRUCTIONS

                        EXHIBIT D - ESCALATION PROCEDURES

                   SCC COMMUNICATIONS CLEARINGHOUSE SERVICES

These escalation procedures are intended as a guideline for ensuring problems
are addressed and resolved in a timely fashion. SCC appreciates Client's
cooperation in abiding by these guidelines.

CLIENT - PROCEDURE

     1.   Client personnel reports a problem to the SCC Clearinghouse Services
          Analyst, relaying pertinent information for initial analysis.

     2.   Depending on the nature of the problem, it may require up to 24 hours
          to resolve the problem. Problem resolution may require more than 24
          hours if resolution depends on other agencies such as the host ALI
          database provider or county coordinator. Client personnel continue to
          monitor the trouble and provide additional information necessary for
          resolution.

     3.   If Client does not receive response to --or- resolution of a severity
          level 1 or 2 problem within the times specified in the Statement of
          Work, the Client may invoke escalation procedures.

          o Client invokes escalation procedures by contacting the Clearinghouse
            Services Supervisor.

          o If Clearinghouse Services Supervisor does not respond or provide
            resolution to problem within 4 hours, Client may escalate to
            Clearinghouse Services Manager.

          o If Clearinghouse Services Manager does not respond or provide
            resolution to problem within 24 hours, Client may escalate to Vice
            President, NDSC Operations.

ESCALATION CONTACT LIST

The following Clearinghouse Services Escalation list can be used for any issues
requiring immediate action.

<TABLE>
<CAPTION>
                                                                 Telephone Number  Pager
                                                                 -------------------------------------------
<S>            <C>                 <C>                           <C>            <C>            <C>
1st Tier       Assigned Analyst    Clearinghouse Analyst         303-581-XXXX
               Assigned Analyst    Clearinghouse Analyst         303-581-XXXX

2nd Tier       Susie Anderson      Clearinghouse Supervisor      303-581-5618   800-724-3722   Pin #911-0048

3rd Tier       Teri Depuy          Director, Data Operations     303-581-5621   800-724-3722   Pin #911-0003
               Keith Collins       Account Manager               303-581-6088   800-724-3722   Pin #911-0084

4th Tier       Mike Wright         VP NDSC Operations            303-581-5624   800-724-3722   Pin #911-0051
</TABLE>

The 24 hour Service Center Emergency Number is 303-581-5782. This number should
be used outside of normal business hours. The Service Center will log the call
and other pertinent information, and notify the appropriate Clearinghouse
Services personnel. Every attempt will be made to answer questions and provide
resolution when the call is received, however, call back situations may occur.

CUSTOMER, EXHIBIT D                 PAGE D-1                             5/14/99
<PAGE>   52

                                     E9-1-1

                                  CLEARINGHOUSE

                                    SERVICES

                                   EXHIBIT E

                                LETTER OF AGENCY

                                   [SCC LOGO]

                               6285 Lookout Road
                      Boulder, Colorado, U.S.A. 80301-3343
                                 (303) 581-5600

<PAGE>   53

                                   EXHIBIT E

                                LETTER OF AGENCY

     This Letter Of Agency made this 22nd day of March, 1999, and is by and
between SCC Communications Corp. a Delaware Corporation having its principal
offices located at 6285 Lookout Road, Boulder, Colorado 80301 and Advanced
TelCom Group, Inc. ("ATG"), a commercial business enterprise having its
principal offices located at 100 Stony Point Road, Suite 130, Santa Rosa,
California 95401.

     ATG hereby authorizes SCC Communications Corp. to act on its behalf for its
use and benefit according to the following instructions:

1. SCC Communications Corp. has agreed to provide certain 911 database
coordination services ("Services" or the "Service") for ATG. SCC's authority to
provide these services is granted by the provisions in the parties Services
Agreement. In order for SCC Communications Corp. to perform such Services, ATG
grants to SCC Communications Corp. the authority to ask for and request
maintaining ATG's rights to the Telephone Subscriber Information, Master Street
Address Guide (MSAG) Information, and daily telephone company service order
update activity used for the purpose of responding to requests for emergency
services for the Local Exchange Provider (LEC) and Emergency Service Provider.
Subscriber information includes, but is not limited to, the collection of names,
addresses, and telephone numbers (whether published, non-published, listed, or
non-listed) for customers of the LEC and Emergency Service Provider, and any
other information associated with and/or supporting the MSAG data for ATG's
customers.

2. SCC Communications Corp. has the authority to procure, receive and collect
from the LEC, the Subscriber Information, and MSAG Information available by law
to ATG for the purpose of providing the E9-1-1 Services. Except as provided that
in paragraph 5.6 of the parties' Service Agreement, SCC Communications Corp. is
responsible for all costs associated with such procurement.

3. SCC Communications Corp. has the authority, on ATG's behalf and as granted by
the provisions specifically provided in the parties' Service Agreement, to
acquire, maintain, and manage the Subscriber Information and MSAG Information
available by law to ATG for the purpose of performing the Services pursuant to
and limited by the provisions of the Services Agreement to which this letter of
agency relates.

4. SCC Communications Corp. hereby agrees to perform its duties hereunder in a
diligent manner, and it is understood by SCC Communications Corp. that ATG may
revoke this letter of agency at any time upon the providing written notice. This
authorization shall remain in force and effect until written notice of
revocation is executed by ATG or by termination of the Services Agreement to
which this letter of agency relates.

SCC COMMUNICATIONS CORP.                     ADVANCED TELCOM GROUP, INC.:

/s/ GEORGE SCHULZE                           /s/ DANIEL L. CRUZ
-----------------------------------          -----------------------------------
Signed                                       Signed

    George Schulze                               Daniel L. Cruz
-----------------------------------          -----------------------------------
By                                           By

    VP Sales                                     VP and Chief Systems Officer
-----------------------------------          -----------------------------------
Title                                        Title

               SCC
        -----------------
           TAR 3/22/99
        -----------------
        ATTORNEY APPROVAL

--------------------------------------------------------------------------------
                            CUSTOMER/SCC PROPRIETARY
                        Use pursuant to SCC Instructions<PAGE>   1

                                                                Stony Point Lake
BASIC LEASE
INFORMATION

                                                                   EXHIBIT 10.16

                      DATE        April 26, 1999

                  LANDLORD        Stony Point East,
                                  a California general partnership

                    TENANT        Advanced Telcom Group,
                                  a Delaware corporation

                  PREMISES        110 Stony Point Road
                                  Suite: 200 - Approximately 37,568 rentable
                                  square feet

             TYPE OF LEASE        Full service

                       USE        General office, including, without limitation,
                                  sales marketing and customer service, and for
                                  no other use or purpose

                      TERM        Ten (10) Years

                 ESTIMATED
         COMMENCEMENT DATE        January 15, 2000

         INITIAL BASE RENT        Monthly: [*] per rentable square foot
                                  Annually: [*] per rentable square foot

        OPERATING EXPENSES        Tenant's Percentage Share:  56.2%
                                  Base Year: 2000

          SECURITY DEPOSIT        Cash in the amount of [*]

        REAL ESTATE BROKER        None

       ADDRESS FOR NOTICES        Landlord:  131 Stony Circle, Suite 480
                                             Santa Rosa, CA 95401

                                  Tenant:    110 Stony Point Road, Suite 200
                                             Santa Rosa, CA 95401

       TENANT IMPROVEMENTS        Landlord will provide tenant improvements as
                                  shown on Exhibit B-3

                  EXHIBITS        A, B, B-1, B-2, B-3, C, D, E, F, G and
                                  Addendum

SIGNATURES:

 /s/ C.G. RANDOLPH                           /s/ JAMES BRECHT
-----------------------------------        -------------------------------------
Tenant                                     Landlord

-----------------------
[*]  Information redacted pursuant to a confidential treatment request
     throughout this exhibit.
<PAGE>   2

TABLE OF CONTENTS

<TABLE>
<CAPTION>
Section                                                                          Page
-------                                                                          ----
<S>                                                                              <C>
    1.     Premises                                                                1
    2.     Term                                                                    1
    3.     Rent                                                                    1
    4.     Base Rent                                                               2
    5.     Additional Rent - Annual Rent Adjustments/Operating Expenses            2
    6.     Proration of Rent                                                       3
    7.     Tenant Improvements                                                     3
    8.     Use of the Premises                                                     3
    9.     Alterations                                                             4
    10.    Repairs                                                                 5
    11.    Damage or Destruction                                                   5
    12.    Eminent Domain                                                          5
    13.    Indemnity and Insurance                                                 6
    14.    Assignment or Sublet                                                    7
    15.    Default                                                                 7
    16.    Landlord's Right to Perform Tenant's Covenants                          8
    17.    Security Deposit                                                        8
    18.    Surrender of Premises                                                   8
    19.    Holding Over                                                            9
    20.    Access to Premises                                                      9
    21.    Signs                                                                   9
    22.    Waiver of Subrogation                                                   9
    23.    Subordination                                                           9
    24.    Transfer of the Property                                               10
    25.    Estoppel Certificates                                                  10
    26.    Mortgagee Protection                                                   10
    27.    Attorneys' Fees                                                        10
    28.    Brokers                                                                10
    29.    Parking                                                                10
    30.    Utilities and Services                                                 11
    31.    Tenant Placement                                                       11
    32.    Acceptance                                                             11
    33.    Use of Building Name                                                   11
    34.    Recording                                                              11
    35.    Quitclaim                                                              11
    36.    Notices                                                                11
    37.    Landlord's Exculpation                                                 12
    38.    Additional Structures                                                  12
    39.    General                                                                12
</TABLE>

                                       i
<PAGE>   3

EXHIBITS

Exhibit A      Site Plan
Exhibit B      Work Letter
Exhibit B-1    Building Shell/Site Work Working Drawings
Exhibit B-2    Interior Improvement Working Drawings
Exhibit B-3    Tenant Preliminary Design and Tenant Improvement Specifications
Exhibit C      Commencement Date Memorandum
Exhibit D      Rules & Regulations
Exhibit E      Utilities and Services
Exhibit F      Hazardous Substance Disclosure
Addendum to Lease

<PAGE>   4

THIS LEASE, which is effective as of the date set forth in the Basic Lease
Information, is entered by Landlord and Tenant, as set forth in the Basic Lease
Information. Terms which are capitalized in this Lease shall have the meanings
set forth in the Basic Lease Information.

1.      PREMISES.

        Landlord leases to Tenant, and Tenant leases from Landlord, the Premises
        described in the Basic Lease Information, together with the right in
        common to use the Common Areas of the Building and the Property (as
        shown in Exhibit A). The Common Areas shall mean the areas and
        facilities within the Building and the Property provided and designated
        by Landlord for the general use, convenience or benefit of Tenant and
        other tenants and occupants of the Building (e.g., restroom's;
        janitorial, telephone and electrical closets; and unreserved parking
        areas). Landlord reserves the right to enlarge, reduce, change or
        otherwise alter the Common Areas of the Building and the Property at any
        time during the term of this Lease. In addition, Landlord shall have the
        right to close temporarily any portion of the Common Areas to make
        repairs or changes or to prevent the acquisition of public rights in the
        Common Areas. [SEE ADDENDUM]

2.      TERM.

        a.      Lease Term. The Term of this Lease shall commence on the
                Commencement Date (as defined in Subsection 2.b.) and, unless
                terminated on an earlier date in accordance with the terms of
                this Lease, shall extend for the period (i.e., Term) specified
                in the Basic Lease Information.

        b.      Commencement Date. The "Commencement Date" of this Lease shall
                be the earliest to occur of the following, as reasonably
                determined by Landlord: [Intentionally deleted] [SEE ADDENDUM]

        c.      Premises Not Delivered. If, for any reason, Landlord cannot
                deliver possession of the Premises to Tenant by the Estimated
                Commencement Date (as set forth in the Basic Lease Information),
                (i) Tenant shall not be obligated to pay Rent until the
                Commencement Date; (ii) the Term shall not be extended; (iii)
                the failure shall not affect the validity of this Lease, or the
                obligations of Tenant under this Lease; and (iv) Landlord shall
                not be subject to any liability.

        d.      Commencement Date Memorandum. When the Commencement Date is
                determined, the parties shall execute a Commencement Date
                Memorandum, in the form attached hereto as Exhibit C, setting
                forth the Commencement Date and the expiration date ("Expiration
                Date") of this Lease. [SEE ADDENDUM]

        e.      Early Entry. If Tenant is permitted to enter the Premises prior
                to the Commencement Date for the purposes of fixturing or any
                purpose other than occupancy permitted by Landlord, the entry
                shall be subject to all the terms and provisions of this Lease,
                except that the payment of Rent shall commence as of the
                Commencement Date.

        f.      Adjustment for Calendar Month. If the Commencement Date is not
                the first day of a calendar month, the Term shall be deemed to
                have commenced on the first day of the next succeeding calendar
                month, for purposes of determining any and all renewal dates,
                the Expiration Date, notice periods for any options, the
                effective date of any increases in Rent (defined in Section 3),
                and any other provisions relating to the Term.

3.      RENT.

        a.      Definitions. As used in this Lease, the term "Rent" shall
                include: (i) the Base Rent; (ii) Tenant's Percentage Share of
                the total dollar increase, if any, in the Operating Expenses
                paid or incurred by Landlord during the calendar year over the
                Operating Expenses paid or incurred by Landlord during the Base
                Year (as set forth in the Basic Lease Information), and (iii)
                all other amounts which Tenant is obligated to pay under the
                terms of this Lease. All amounts of money payable by Tenant to
                Landlord shall be paid without prior notice or demand, deduction
                or offset. If any installment of Rent is not paid by the fifth
                (5th) day of the month, Tenant shall pay to Landlord a late
                payment charge equal to five percent (5%) of the amount of the
                delinquent installment, in addition to the installment of Rent
                then owing, regardless of whether a notice of default or notice
                of termination has been given by Landlord. In addition to the
                five percent (5%) late charge, any Rent or other amounts owing
                hereunder which are not paid within five (5) days after the date
                they are due shall thereafter bear interest at the rate
                ("Interest Rate") which is the [*]. [SEE ADDENDUM]

-----------------------
[*]  Information redacted pursuant to a confidential treatment request
     throughout this exhibit.
                                        1
<PAGE>   5

        b.      Accord and Satisfaction. No payment by Tenant or receipt by
                Landlord of a lesser amount than any installment or payment of
                Rent due shall be deemed to be other than on account of the
                amount due, and no endorsement or statement on any check or
                payment of Rent shall be deemed an accord and satisfaction.
                Landlord may accept such check or payment without prejudice to
                Landlord's right to recover the balance of such installment or
                payment of Rent, or pursue any other remedies available to
                Landlord. [SEE ADDENDUM]

4.      BASE RENT.

        a.      Initial Base Rent. Tenant shall pay Base Rent to Landlord (or
                other entity designated by Landlord), in advance, on the first
                day of each calendar month of the Term, at Landlord's address
                for notices (as set forth in the Basic Lease Information) or at
                such other address as Landlord may designate. The Initial Base
                Rent shall be the amount set forth in the Basic Lease
                Information. Upon executing this Lease, Tenant shall pay an
                amount equal to the monthly Base Rent stated on the Basic Lease
                Information, which shall be credited against the Base Rent due
                at the beginning of the Term. [SEE ADDENDUM]

        b.      [Intentionally deleted] [SEE ADDENDUM]

5.      ADDITIONAL RENT - ANNUAL RENT ADJUSTMENTS/OPERATING EXPENSES.

        a.      Increase in Operating Expenses. Rent shall include Tenant's
                Percentage Share of the total dollar increase, if any, in the
                Operating Expenses paid or incurred by Landlord during the
                calendar year over the Operating Expenses paid or incurred by
                Landlord during the Base Year.

        b.      Operating Expenses. The term "Operating Expenses" shall include
                all reasonable expenses and costs of every kind and nature which
                Landlord shall pay or become obligated to pay because of or in
                connection with the ownership and operation of the Building,
                Common Areas and Premises, surrounding property and supporting
                facilities. Operating Expenses shall include, without
                limitation, the following: (i) all impositions relating to the
                Building, Common Areas and Premises, including the Real Property
                Taxes (as defined in Section 5.d.); (ii) premiums for insurance
                relating to the Building, Common Areas and Premises as set forth
                in Sections 13.b., 13.d. and 13.h.; (iii) wages, salaries and
                related expenses and benefits of all on-site and off-site
                employees engaged in operation, maintenance and security; (iv)
                all supplies, materials and equipment rental used in operations;
                (v) all maintenance, janitorial, security and service costs;
                (vi) a management fee; (vii) legal and accounting expenses,
                including the cost of audits by certified public accountants;
                (viii) repairs, replacements and general maintenance (excluding
                those paid for by proceeds of insurance or other parties, and
                alterations attributable solely to other tenants of the
                Buildings; (ix) all maintenance costs relating to the Building
                and Common Areas, including sidewalks, landscaping, service
                areas, mechanical rooms, parking areas, Building exterior and
                driveways; [Intentionally deleted] (xi) all other operating,
                management and other expenses incurred by Landlord in connection
                with operation of the Building and Common Areas; (xii) all
                charges for heat, water, gas, electricity and other utilities
                used or consumed in the Building and Common Areas, entranceways,
                sidewalks, etc.; and (xiii) ground rent payments, if any. [SEE
                ADDENDUM]

        c.      Monthly Increments; Adjustment. Prior to the commencement of
                each of Landlord's accounting years, Landlord shall estimate the
                amount of the Operating Expenses payable by Tenant for the next
                accounting year pursuant to

                                        2
<PAGE>   6
                this Section. Tenant shall pay to Landlord, on the first of each
                month, in advance, one-twelfth (1/12) of Landlord's estimate.
                Within ninety (90) days after (or as soon thereafter as
                possible) the close of each accounting year after which this
                Lease remains in effect, Landlord shall provide Tenant with a
                statement [Intentionally deleted]. Landlord, in Landlord's sole
                discretion, shall have the right to reduce the Operating
                Expenses to limit the impact (e.g., year to year fluctuations)
                resulting from extraordinary or unusual expenses or costs. If
                Tenant has overpaid the amount of Operating Expenses owing
                pursuant to this Section, Landlord shall credit the overpayment
                to Tenant within thirty (30) days after Tenant's receipt of
                Landlord's statement; provided, however, that the Rent owed by
                Tenant shall never be less than the Base Rent stated in Section
                4. If Tenant has underpaid the amount of Operating Expenses
                owing pursuant to this Section, Tenant shall pay the amount of
                the underpayment to Landlord, as Additional Rent, within thirty
                (30) days after Tenant's receipt of Landlord's statement. If
                less than one hundred percent (100%) of the rentable area of the
                Building is occupied, Operating Expenses shall be adjusted to
                equal Landlord's reasonable estimate of Operating Expenses if
                one hundred percent (100%) of the total rentable area of the
                Building were occupied. [SEE ADDENDUM]

        d.      Definition of Real Property Taxes. The term "Real Property
                Taxes" shall mean any ordinary or extraordinary form of
                assessment or special assessment, license fee, rent tax, levy,
                penalty (if a result of Tenant's delinquency), or tax, other
                than net income, estate, succession, inheritance, transfer or
                franchise taxes, imposed by any authority having the direct or
                indirect power to tax, or by any city, county, state or federal
                government for any maintenance or improvement or other district
                or division thereof. The term shall include all transit charges,
                housing fund assessments, real estate taxes and all other taxes
                relating to the Premises, Building and/or Property, all other
                taxes which may be levied in lieu of real estate taxes
                (including any increases resulting from changes of ownership),
                all assessments, assessment bonds, levies, fees, and other
                governmental charges (including, but not limited to, charges for
                traffic facilities, improvements, child care, water services
                studies and improvements, and fire services studies and
                improvements) for amounts necessary to be expended because of
                governmental orders, whether general or special, ordinary or
                extraordinary, unforeseen as well as foreseen, of any kind and
                nature for public improvement, services, benefits or any other
                purposes which are assessed, levied, confirmed, imposed or
                become a lien upon the Premises, Building or Property or become
                payable during the Term. [SEE ADDENDUM]

        e.      Taxes on Tenant Improvements and Personal Property.
                Notwithstanding any other provision hereof, Tenant shall pay the
                full amount of any increase in Real Property Taxes during the
                Term resulting from any and all alterations and tenant
                improvements of any kind whatsoever placed in, on or about the
                Premises [Intentionally deleted]. Tenant shall pay, prior to
                delinquency, all taxes assessed or levied against Tenant's
                personal property in, on or about the Premises. When possible,
                Tenant shall cause its personal property to be assessed and
                billed separately from the real or personal property of
                Landlord. [SEE ADDENDUM]

6.      PRORATION OF RENT.

                If the Commencement Date is not the first day of the month, Rent
                shall be prorated on a monthly basis for the fractional month
                during the month which this Lease commences. The termination of
                this Lease shall not affect the obligations of Landlord and
                Tenant pursuant to Subsection 5.c. which are to be performed
                after the termination.

7.      TENANT IMPROVEMENTS.

        [Intentionally deleted] [SEE ADDENDUM]

8.      USE OF THE PREMISES.

        a.      Use. The Premises shall be used solely for the use set forth in
                the Basic Lease Information and for no other use.

        b.      Rules and Regulations; CC&R's. Tenant shall comply with the
                Rules and Regulations attached hereto as Exhibit D. In addition,
                Tenant shall comply with [Intentionally deleted] covenants,
                conditions and restrictions ("CC&R's") applicable to the
                Building, and all rules, regulations and restrictions
                (collectively, "Rules") imposed by any association
                ("Association") formed pursuant to the CC&R's. [SEE ADDENDUM]

        c.      Compliance. Tenant, at its sole cost and expense, shall promptly
                comply with all laws, statutes, ordinances and governmental
                rules, regulations or requirements (including the Americans with
                Disabilities Act) now in force or which hereinafter may be in
                force, with the requirements of any board of fire underwriters
                or other similar board now or hereafter constituted, with any
                direction or occupancy certificate issued pursuant to any law by
                any public officer or officers, as well

                                        3
<PAGE>   7

                as the provisions of all recorded documents affecting the
                Premises, insofar as any thereof relate to or affect the
                condition, use or occupancy of the Premises. [SEE ADDENDUM]

        D.      HAZARDOUS SUBSTANCES.

                (i) Hazardous Substances. The term "Hazardous Substances" as
                used in this Lease shall mean any product, substance, chemical,
                material or waste whose presence, nature, quantity and/or
                intensity of existence, use manufacture, disposal,
                transportation, spill, release or effect, either by itself or in
                combination with other materials expected to be on the Premises,
                is either (A) potentially injurious to the public health, safety
                or welfare, the environment or the Premises, (B) regulated or
                monitored by any governmental authority, or (C) a basis for
                liability of Landlord to any governmental agency or third party
                under any applicable statute or common law theory. Hazard
                Substances include, but are not limited to, hydrocarbons,
                petroleum, gasoline, crude oil and any products, by-products or
                fractions thereof.

                (ii) Duty to Inform Landlord. If Tenant knows, or has reasonable
                cause to believe, that a Hazardous Substance, or a condition
                involving or resulting from same, has come to be located in, on,
                under or about the Premises, other than as previously consented
                to by Landlord, Tenant shall immediately give written notice of
                such fact to Landlord. Tenant shall also immediately give
                Landlord a copy of any statement, report, notice, registration,
                application, permit, business plan, license, claim, action or
                proceeding given to, or received from, any governmental
                authority or private party, or persons entering or occupying the
                Premises, concerning the presence, spill, release, discharge of,
                or exposure to, any Hazardous Substance or contamination in, on,
                under or about the Premises.

                (iii) Indemnification. Tenant shall indemnify, protect, defend
                and hold harmless Landlord, its agents, employees, and lenders,
                if any, and the Premises from and against all loss of rents
                and/or damages, liabilities, judgments, costs, claims, liens,
                expenses, penalties, permits and attorneys' and consultants'
                fees arising out of or involving any Hazardous Substances
                brought onto the Premises by or for Tenant or under Tenant's
                control. Tenant's obligations under this Section 8.d. shall
                include, but not be limited to, the effects of any contamination
                or injury to persons, property or the environment created or
                suffered by Tenant, and the cost of investigation (including
                consultants' and attorneys' fees and testing), removal,
                remediation, restoration and/or abatement thereof, or of any
                contamination therein involved, and shall survive the expiration
                or earlier termination of this Lease. No termination,
                cancellation or release agreement entered into by Landlord and
                Tenant shall release Tenant from its obligations under this
                Lease with respect to Hazardous Substances, unless specifically
                so agreed by Landlord in writing at the time of such agreement.
                [SEE ADDENDUM]

9.      ALTERATIONS.

        a.      Permitted Alterations. Tenant shall give Landlord not less than
                ten (10) days' prior written notice of any alteration Tenant
                desires to make to the Premises. Tenant shall not make any
                alteration in, on or about the Premises without the prior
                written consent of Landlord, which consent shall not be
                unreasonably withheld or delayed. Tenant shall comply with all
                rules, laws, ordinances and requirements at the time Tenant
                makes any alteration and shall deliver to Landlord a complete
                set of "as built" plans and specifications for each alteration.
                Tenant shall be solely responsible for maintenance and repair of
                all alterations made by Tenant. As used in this Section 9, the
                term "alteration" shall include any alteration, addition or
                improvement. [SEE ADDENDUM]

        b.      Liens. If, because of any act or omission of Tenant or anyone
                claiming by, through, or under Tenant, any mechanic's lien or
                other lien is filed against the Premises, the Building, the
                Property or against other property of Landlord (whether or not
                the lien is valid or enforceable), Tenant shall, at its own
                expense, cause it to be discharged of record within a reasonable
                time, not to exceed [intentionally deleted] days, after the date
                of the filing. In addition, Tenant shall defend and indemnify
                Landlord and hold it harmless from any and all claims, losses,
                damages, judgments, settlements, costs and expenses, including
                attorneys' fees, resulting from the lien. [SEE ADDENDUM]

        c.      Ownership of Alterations. Any alteration made by Tenant shall
                immediately become Landlord's property. Except as provided in
                Subsection 9.d., Landlord may require Tenant, at Tenant's sole
                expense and by the end of the Term, to remove any alterations
                made by Tenant and to restore the Premises to its condition
                prior to the alteration. [SEE ADDENDUM]

        d.      Request Regarding Removal Obligation. At the time that Tenant
                requests Landlord's consent to any alteration, Tenant may
                request that Landlord notify Tenant if Landlord will require
                Tenant, at Tenant's sole expense, to remove any or all of the
                alteration by the end of the Term, and to restore the Premises
                to its condition prior to the alteration.

                                       4
<PAGE>   8
10.     REPAIRS.

        Tenant, at all times during the Term and at Tenant's sole cost and
        expense, shall keep the Premises and every part thereof, including all
        telecommunication risers and cabling, in good condition and repair;
        ordinary wear and tear, damage by fire, earthquake, acts of God or the
        elements excepted. Tenant hereby waives all right to make repairs at the
        expense of Landlord or in lieu thereof to vacate the Premises as
        provided in California Civil Code Section 1942 or any other law, statute
        or ordinance now or hereafter in effect. [SEE ADDENDUM]

11.     DAMAGE OR DESTRUCTION.

        a.      Landlord's Obligation to Rebuild. If the Premises are damaged or
                destroyed, Landlord shall promptly and diligently repair the
                Premises unless Landlord has the option to terminate this Lease
                as provided herein, and Landlord elects to terminate.

        b.      Right to Terminate. Landlord shall have the option to terminate
                this Lease if the Premises or the Building is destroyed or
                damaged by fire or other casualty, regardless of whether the
                casualty is insured against under this Lease, if Landlord
                reasonably determines that the repair of the Premises or the
                Building cannot be completed within two hundred seventy (270)
                days after the [intentionally deleted] casualty. If Landlord
                desires to exercise the right to terminate this Lease as a
                result of a casualty, Landlord shall exercise the right by
                giving Tenant written notice of its election to terminate within
                thirty (30) days after the, in which event this Lease shall
                terminate fifteen (15) days after the date of the notice. If
                Landlord does not exercise the right to terminate this Lease,
                Landlord shall promptly commence the process of obtaining
                necessary permits and approvals, and shall commence repair of
                the Premises or the Building as soon as practicable and
                thereafter prosecute the repair diligently to completion, in
                which event this Lease shall continue in full force and effect.
                [SEE ADDENDUM]

        c.      Limited Obligation to Repair. Landlord's obligation, should
                Landlord elect or be obligated to repair or rebuild, shall be
                limited to the Building shell and any tenant improvements which
                are constructed and paid for by Landlord pursuant to Exhibit B.
                Tenant, at its option and expense, shall replace or fully repair
                all trade fixtures, equipment and other improvements, including
                all telecommunication risers and cabling, installed by Tenant
                and existing at the time of the damage or destruction.

        d.      Abatement of Rent. In the event of any damage or destruction to
                the Premises which does not result in termination of this Lease,
                the Base Rent shall be temporarily abated proportionately to the
                degree the Premises are untenantable as a result of the damage
                or destruction, commencing from the date of the damage or
                destruction and continuing during the period required by
                Landlord to substantially complete its repair and restoration of
                the Premises; provided, however, that nothing herein shall
                preclude Landlord from being entitled to collect the full amount
                of any rent loss insurance proceeds. Tenant shall not be
                entitled to any compensation or damages from Landlord for loss
                of the use of the Premises, damage to Tenant's personal property
                or any inconvenience occasioned by any damage, repair or
                restoration. Tenant hereby waives the provisions of Section
                1932, Subdivision 2, and Section 1933, Subdivision 4, of the
                California Civil Code, and the provisions of any similar law
                hereafter enacted.

        e.      Damage Near End of Term and Extensive Damage. In addition to the
                rights to termination under Subsection 11.b., Landlord shall
                have the right to cancel and terminate this Lease as of the date
                of the occurrence of destruction or damage if the Premises or
                the Building is [intentionally deleted] made untenantable during
                the last twelve (12) months of the Term. Landlord shall give
                notice of its election to terminate this Lease under this
                Subsection 11.e. within thirty (30) days after Landlord
                determines that the damage or destruction would require more
                than six (6) months to repair. If Landlord does not elect to
                terminate this Lease, the repair of the damage shall be governed
                by Subsection 11.a. or 11.b., as the case may be. [SEE ADDENDUM]

               f. Insurance Proceeds. If this Lease is terminated, Landlord may
               keep all the insurance proceeds resulting from the damage, except
               for those proceeds which specifically insured Tenant's personal
               property and trade fixtures. [SEE ADDENDUM]

12.     EMINENT DOMAIN.

        If all or any part of the Premises is taken for public or quasi-public
        use by a governmental authority under the power of eminent domain or is
        conveyed to a governmental authority in lieu of such taking, and if
        [intentionally deleted] the taking or conveyance causes the remaining
        part of the Premises to be untenantable and inadequate for use by Tenant
        for the purpose for which they were leased, then Tenant, at its option
        and by giving written notice within fifteen (15) days after the taking,
        may terminate this Lease as of the date Tenant is required to surrender
        possession of the Premises. If a part of the Premises is taken or
        conveyed but the remaining part is tenantable and

                                       5
<PAGE>   9

        adequate for Tenant's use, then this Lease shall be terminated as to the
        part taken or conveyed as of the date Tenant surrenders possession;
        Landlord shall make such repairs, alterations and improvements as may be
        necessary to render the part not taken or conveyed tenantable; and the
        Rent shall be reduced in proportion to the part of the Premises taken or
        conveyed. All compensation awarded for the taking or conveyance shall be
        the property of Landlord without any deduction therefrom for any estate
        of Tenant, and Tenant hereby assigns to Landlord all its right, title
        and interest in and to the award. Tenant shall have the right, however,
        to recover from the governmental authority, but not from Landlord, such
        compensation as may be awarded to Tenant on account of the interruption
        of Tenant's business, moving and relocation expenses and removal of
        Tenant's trade fixtures and personal property.

13.     INDEMNITY AND INSURANCE.

        a.      Indemnity. Tenant shall be responsible for, shall insure
                against, and shall indemnify Landlord and its constituent parts
                and hold them harmless from, any and all liability for any loss,
                damage or injury to person or property occurring in, on or about
                the Premises, and Tenant hereby releases Landlord and its
                constituent parts from any and all liability for the same.
                Tenant's obligation to indemnify Landlord and its constituent
                parts hereunder shall include the duty to defend against any
                claims asserted by reason of any loss, damage or injury, and to
                pay any judgments, settlements, costs, fees and expenses,
                including attorneys' fees, incurred in connection therewith with
                counsel reasonably acceptable to Landlord. [SEE ADDENDUM]

        b.      Fire and Extended Coverage. Landlord shall procure and maintain
                in full force and effect with respect to the Building a policy
                or policies of all risk or "Special Term" insurance for the full
                insurance replacement value thereof.

        c.      Public Liability. Tenant, at its own cost and expense, shall
                keep and maintain in full force and effect during the Term a
                policy or policies of commercial general public liability
                insurance, written by an insurance company approved by Landlord,
                in the form customary to the locality, insuring Tenant's
                activities with respect to the Premises and/or the Building
                against loss, damage or liability for personal injury or death
                of any person or loss or damage to property occurring in, upon
                or about the Premises, covering bodily injury in the amounts of
                Two Million Dollars ($2,000,000) per person and Two Million
                Dollars ($2,000,000) per occurrence, and covering property
                damage in the amount of One Million Dollars ($1,000,000);
                provided, however, that if, at any time during the Term, Tenant
                shall have in full force and effect a blanket policy of public
                liability insurance with the same coverage for the Premises as
                described above, as well as coverage of other premises and
                properties of Tenant, or in which Tenant has some interest, the
                blanket insurance shall satisfy the requirement hereof.

        d.      Rental Abatement Insurance. Landlord may keep and maintain in
                full force and effect during the Term rental abatement insurance
                against abatement or loss of Rent in case of fire or other
                casualty, in an amount at least equal to the amount of Rent
                payable by Tenant during one (1) year of the Term, as reasonably
                determined by Landlord.

        e.      Insurance Certificates. Tenant shall furnish to Landlord, upon
                the Commencement Date and thereafter within thirty (30) days
                prior to the expiration of each policy, a certificate of
                insurance issued by the insurance carrier of each policy of
                insurance carried by Tenant pursuant to this Section 13. The
                certificates shall expressly provide that the policies shall not
                be cancelable or subject to reduction of coverage or otherwise
                be subject to modification except after thirty (30) days' prior
                written notice to the parties named as additional insured.
                Landlord, its successors and assigns, and any nominee of
                Landlord holding any interest in the Premises, including,
                without limitation, any ground lessor or the holder of any fee
                or leasehold mortgage, shall be named as an additional insured
                under each policy of insurance maintained by Tenant pursuant to
                this Lease.

        f.      Tenant's Failure. Tenant's failure to maintain any insurance
                required by this Lease, shall be a default hereunder, and Tenant
                shall be liable for any loss or costs resulting from the
                failure.

        g.      Tenant's Property and Fixtures. Tenant shall assume the risk of
                theft or damage to any furniture, equipment, machinery, goods,
                supplies or fixtures which are or remain the property of Tenant,
                or as to which Tenant retains the right of removal from the
                Premises.

        h.      Earthquake and Flood Insurance. In addition to any other
                insurance policies carried by Landlord in connection with the
                Building, Landlord may elect to procure and maintain in full
                force and effect during the Term with respect to the Building a
                policy of earthquake/volcanic action and flood and/or surface
                water insurance, including rental value insurance against
                abatement or loss of rent in the case of damage or loss covered
                under the earthquake/volcanic and flood and/or surface water
                insurance, in an amount equal to one hundred percent (100%) of
                the full insurance replacement value (including debris removal
                and demolition) of the Building. [SEE ADDENDUM]

                                       6
<PAGE>   10

14.     ASSIGNMENT OR SUBLET.

        If Tenant desires to assign this Lease or to sublet the Premises, or any
        part thereof, Tenant shall give to Landlord written notice of its intent
        at least [Intentionally deleted] days in advance of the date on which
        Tenant desires to assign or sublet the Premises. Landlord shall have
        thirty (30) days after receipt of Tenant's written notice within which
        to notify Tenant in writing that Landlord elects to [Intentionally
        deleted] permit Tenant to assign this Lease or sublease the Premises,
        subject, however, to Landlord's prior written approval, which approval
        shall not be unreasonably withheld. Landlord and Tenant agree that it
        shall be reasonable for Landlord to withhold its consent if Landlord
        reasonably determines that the proposed assignee or sublessee (A) does
        not have the financial ability to fulfill all of the obligations of
        Tenant under this Lease, (B) is to use Hazardous Substances in type or
        quantity in excess of those used by Tenant, (C) will potentially
        increase the Operating Expenses, (D) is a governmental entity,
        [Intentionally deleted]. Upon any proposed assignment or sublet, Tenant
        shall enter into a written agreement with Landlord requiring that
        [Intentionally deleted] of the excess moneys due to Tenant under the
        assignment or sublet over the Rent to be required to be paid by Tenant
        hereunder shall be paid to Landlord. No consent by Landlord to any
        assignment or sublet shall be deemed to be a consent to a use not
        permitted under this Lease, to any act in violation of this Lease or to
        any subsequent assignment or sublet. No assignment or sublet by Tenant
        shall relieve Tenant of any obligation under this Lease. Any attempted
        assignment or sublet by Tenant in violation of the terms and covenants
        of this Section shall be void. [SEE ADDENDUM]

15.     DEFAULT.

        a.      Tenant's Default. At the option of Landlord, a material breach
                of this Lease by Tenant shall exist if any of the following
                events (severally, "Event of Default"; collectively, "Events of
                Default") shall occur: (i) if Tenant shall have failed to pay
                Rent, including Tenant's Percentage Share of increased Operating
                Expenses, or any other sum required to be paid hereunder
                [Intentionally deleted], together with interest at the Interest
                Rate, from the date the amount became due through the date of
                payment, inclusive; (ii) if Tenant shall have failed to perform
                any term, covenant or condition of this Lease except those
                requiring the payment of money, and Tenant shall have failed to
                cure the breach within [Intentionally deleted] days after
                written notice from Landlord if the breach could reasonably be
                cured within the [Intentionally deleted] day period; provided,
                however, if the failure could not reasonably be cured within the
                [Intentionally deleted] day period, then Tenant shall not be in
                default unless it has failed to promptly commence and thereafter
                continue to make diligent and reasonable efforts to cure the
                failure as soon as practicable as reasonably determined by
                Landlord; (iii) if Tenant shall have its assets for the benefit
                of its creditors; (iv) if the [Intentionally deleted]
                sequestration of, attachment of, or execution on, any material
                part of the property of Tenant or on any property essential to
                the conduct of Tenant's business shall have occurred, and Tenant
                shall have failed to obtain a return or release of the property
                within thirty (30) days thereafter, or prior to sale pursuant to
                any sequestration, attachment or levy, whichever is earlier; (v)
                if Tenant shall have [Intentionally deleted] abandoned
                [Intentionally deleted] the Premises; (vi) if a court shall have
                made or entered any decree or order adjudging Tenant to be
                insolvent, or approving as properly filed a petition seeking
                reorganization of Tenant, or directing the winding up or
                liquidation of Tenant, and the decree or order shall have
                continued for a period of thirty (30) days; (vii) if Tenant
                shall make or suffer any transfer which constitutes a fraudulent
                or otherwise avoidable transfer under any provision of the
                federal Bankruptcy Laws or any applicable state law; or (viii)
                if Tenant shall have failed to comply with the provisions of
                Sections 23 or 25 of this Lease. An Event of Default shall
                constitute a default under this Lease. [SEE ADDENDUM]

        b.      Remedies Upon Tenant's Default. Upon an Event of Default,
                Landlord shall have the following remedies, in addition to all
                other rights and remedies provided by law, equity, statute or
                otherwise provided in this Lease, to which Landlord may resort
                cumulatively or in the alternative:

                (i)     Landlord may continue this Lease in full force and
                        effect, and this Lease shall continue in full force and
                        effect as long as Landlord does not terminate Tenant's
                        right to possession, and Landlord shall have the right
                        to collect Rent when due [Intentionally deleted]. No act
                        by Landlord allowed by this Subsection (i) shall
                        terminate this Lease unless Landlord notifies Tenant in
                        writing that Landlord elects to terminate this Lease.
                        [SEE ADDENDUM]

                (ii)    Landlord may terminate Tenant's right to possession of
                        the Premises at any time by giving written notice to
                        that effect. No act by Landlord other than giving
                        written notice to Tenant shall terminate this Lease.
                        Acts of

                                       7
<PAGE>   11

                        maintenance, efforts to relet the Premises or the
                        appointment of a receiver on Landlord's initiative to
                        protect Landlord's interest under this Lease shall not
                        constitute a termination of Tenant's right to
                        possession. On termination, Landlord shall have the
                        right to remove all personal property of Tenant and
                        store it at Tenant's cost and to recover from Tenant as
                        damages: (a) the worth at the time of award of unpaid
                        Rent and other sums due and payable which had been
                        earned at the time of termination; plus (b) the worth at
                        the time of award of the amount by which the unpaid Rent
                        and other sums due and payable which would have been
                        payable after termination until the time of award
                        exceeds the amount of the Rent loss that Tenant proves
                        could have been reasonably avoided; plus (c) the worth
                        at the time of award of the amount by which the unpaid
                        Rent and other sums due and payable for the balance of
                        the Term after the time of award exceeds the amount of
                        the Rent loss that Tenant proves could be reasonably
                        avoided; plus (d) any other amount necessary to
                        compensate Landlord for all the detriment proximately
                        caused by Tenant's failure to perform Tenant's
                        obligations under this Lease, or which, in the ordinary
                        course of things, would be likely to result therefrom,
                        including, without limitation, any costs or expenses
                        incurred by Landlord: (1) in retaking possession of the
                        Premises, including reasonable attorneys' fees and costs
                        therefor; (2) maintaining or preserving the Premises for
                        reletting to a new tenant, including repairs or
                        alterations to the Premises for the reletting; (3)
                        leasing commissions; (4) any other costs necessary or
                        appropriate to relet the Premises; and (5) at Landlord's
                        election, such other amounts in addition to or in lieu
                        of the foregoing as may be permitted from time to time
                        by the laws of the State of California.

                        The "worth at the time of award" of the amounts referred
                        to in Subsections (ii)(a) and (ii)(b) is computed by
                        allowing interest at the lesser of [Intentionally
                        deleted] per annum or the maximum rate permitted by law,
                        on the unpaid Rent and other sums due and payable from
                        the termination date through the date of award. The
                        "worth at the time of award" of the amount referred to
                        in Subsection (ii)(c) is computed by discounting the
                        amount at the discount rate of the Federal Reserve Bank
                        of San Francisco at the time of award, plus one percent
                        (1%). Tenant waives redemption or relief from forfeiture
                        under California Code of Civil Procedure Sections 1174
                        and 1179, or under any other present or future law, if
                        Tenant is evicted or Landlord takes possession of the
                        Premises by reason of any default of Tenant hereunder.
                        [SEE ADDENDUM]

      c.    Landlord's Default. Landlord shall not be deemed to be in default in
            the performance of any obligation required to be performed by
            Landlord hereunder unless and until Landlord has failed to perform
            the obligation within thirty (30) days after receipt of written
            notice by Tenant to Landlord specifying wherein Landlord has failed
            to perform the obligation; provided, however, that if the nature of
            Landlord's obligation is such that more than thirty (30) days are
            required for its performance, then Landlord shall not be deemed to
            be in default if Landlord shall commence the performance within the
            thirty (30) day period and thereafter shall diligently prosecute the
            same to completion.

16.   LANDLORD'S RIGHT TO PERFORM TENANT'S COVENANTS.

      If Tenant shall at any time fail to make any payment or perform any other
      act on its part to be made or performed under this Lease, Landlord may,
      but shall not be obligated to, make the payment or perform any other act
      to the extent Landlord may deem desirable and, in connection therewith,
      pay expenses and employ counsel. Any payment or performance by Landlord
      shall not waive or release Tenant from any obligations of Tenant under
      this Lease. All sums so paid by Landlord, and all penalties, interest and
      costs in connection therewith, shall be due and payable by Tenant on the
      next day after any payment by Landlord, together with interest thereon at
      the Interest Rate, from that date to the date of payment thereof by Tenant
      to Landlord, plus collection costs and attorneys' fees. Landlord shall
      have the same rights and remedies for the nonpayment thereof as in the
      case of default in the payment of Rent. [SEE ADDENDUM]

17.   SECURITY DEPOSIT.

      Tenant has deposited with Landlord the Security Deposit, in the amount
      specified in the Basic Lease Information, as security for the full and
      faithful performance of every provision of this Lease to be performed by
      Tenant. If Tenant defaults with respect to any provision of this Lease,
      Landlord may use, apply or retain all or any part of the Security Deposit
      for the payment of any Rent or other sum in default, for the payment of
      any amount which Landlord may expend or become obligated to expend by
      reason of Tenant's default, or to compensate Landlord for any loss or
      damage which Landlord may suffer by reason of Tenant's default. If any
      portion of the Security Deposit is used or applied, Tenant shall deposit
      with Landlord, within ten (10) days after written demand therefor, cash in
      an amount sufficient to restore the Security Deposit to its original
      amount. Landlord shall not be required to keep the Security Deposit
      separate from its general funds. [Intentionally deleted] [SEE ADDENDUM]

18.   SURRENDER OF PREMISES.

      By taking possession of the Premises, Tenant shall be deemed to have
      accepted the Premises, Building, and the Property in good, clean
      [Intentionally deleted] condition [Intentionally deleted]. On the
      expiration or early termination of this

                                       8
<PAGE>   12

      Lease, Tenant shall surrender the Premises to Landlord in its condition as
      of the Commencement Date, normal wear and tear excepted. Tenant shall
      remove from the Premises all of Tenant's personal property, trade fixtures
      and any alterations required to be removed pursuant to Section 9 of this
      Lease. Tenant shall repair damage or perform any restoration work required
      by the removal. If Tenant fails to remove any personal property, trade
      fixtures or alterations after the end of the Term, Landlord may remove the
      property and store it at Tenant's expense, including interest at the
      Interest Rate. If the Premises are not so surrendered at the termination
      of this Lease, Tenant shall indemnify Landlord against all loss or
      liability resulting from delay by Tenant in so surrendering the Premises,
      including, without limitation, any claims made by any succeeding tenant,
      losses to Landlord due to lost opportunities to lease to succeeding
      tenants, and attorneys' fees and costs. [SEE ADDENDUM]

19.   HOLDING OVER.

      If Tenant remains in possession of all or any part of the Premises after
      the expiration of the Term or the termination of this Lease, the tenancy
      shall be month-to-month only and shall not constitute a renewal or
      extension for any further term. In such event, Base Rent shall be
      increased in an amount equal to [*] of the Base Rent during the last month
      of the Term (including any extensions), and any other sums due under this
      Lease shall be payable in the amount, and at the times, specified in this
      Lease. The month-to-month tenancy shall be subject to every other term,
      condition, covenant and agreement contained in this Lease and Tenant shall
      vacate the Premises immediately upon Landlord's request. [SEE ADDENDUM]

20.   ACCESS TO PREMISES.

      Tenant shall permit Landlord and its agents to enter the Premises at all
      reasonable times, to inspect the Premises; to post Notices of
      Nonresponsibility and similar notices and to show the Premises to
      interested parties such as prospective mortgagors, purchasers and tenants;
      to make necessary alterations, additions, improvements or repairs either
      to the Premises, the Building, or other premises within the Building; and
      to discharge Tenant's obligations hereunder when Tenant has failed to do
      so within a reasonable time after written notice from Landlord. The above
      rights are subject to reasonable security regulations of Tenant, and to
      the requirement that Landlord shall at all times act in a manner to cause
      the least possible interference with Tenant's operations. [SEE ADDENDUM]

21.   SIGNS.

      The size, design, color, location and other physical aspects of any sign
      in or on the Building shall be subject to the CC&R's (if applicable),
      Rules, Landlord's approval prior to installation, and to any appropriate
      municipal or other governmental approvals. The costs of any permitted
      sign, and the costs of its installation, maintenance and removal, shall be
      at Tenant's sole expense and shall be paid within ten (10) days of
      Tenant's receipt of a bill from Landlord for the costs. [SEE ADDENDUM]

22.   WAIVER OF SUBROGATION.

      Anything in this Lease to the contrary notwithstanding, Landlord and
      Tenant each hereby waives and releases the other of and from any and all
      rights of recovery, claim, action or cause of action against the other,
      its subsidiaries, directors, agents, officers and employees, for any loss
      or damage that may occur in the Premises, the Building or the Property; to
      improvements to the Building or personal property (building contents)
      within the Building; or to any furniture, equipment, machinery, goods and
      supplies not covered by this Lease which Tenant may bring or obtain upon
      the Premises or any additional improvements which Tenant may construct on
      the Premises by reason of fire, the elements or any other cause which is
      required to be insured against under this Lease, regardless of cause or
      origin, including negligence of Landlord or Tenant and their agents,
      subsidiaries, directors, officers and employees, to the extent insured
      against under the terms of any insurance policies carried by Landlord or
      Tenant and in force at the time of any such damage, [Intentionally
      deleted]. Because this Section 22 will preclude the assignment of any
      claim mentioned in it by way of subrogation or otherwise to an insurance
      company or any other person, each party to this Lease agrees immediately
      to give to each of its insurance companies written notice of the terms of
      the mutual waivers contained in this Section 22 and to have the insurance
      policies properly endorsed, if necessary, to prevent the invalidation of
      the insurance coverages by reason of the mutual waivers contained in this
      Section 22.

23.   SUBORDINATION.

      a.    SUBORDINATE NATURE. Except as provided in Subsection b., this Lease
            is subject and subordinate to all ground and underlying leases,
            mortgages and deeds of trust which now or may hereafter affect the
            Property, the Building or the Premises, to the CC&R's, and to all
            renewals, modifications, consolidations, replacements and extensions
            thereof. Within [Intentionally deleted] days after Landlord's
            written request, Tenant shall execute any and all documents required
            by Landlord, the lessor under any ground or underlying lease
            ("Lessor"), or the holder or holders of any mortgage or deed of
            trust ("Holder") to make this Lease subordinate to the lien of any
            lease, mortgage or deed of trust, as the case may be. [SEE ADDENDUM]

-----------------------
[*]  Information redacted pursuant to a confidential treatment request
     throughout this exhibit.

                                       9
<PAGE>   13

      b.    POSSIBLE PRIORITY OF LEASE. If a Lessor or a Holder advises Landlord
            that it desires or requires this Lease to be prior and superior to a
            lease, mortgage or deed of trust, Landlord may notify Tenant. Within
            [Intentionally deleted] days of Landlord's notice, Tenant shall
            execute, have acknowledged and deliver to Landlord any and all
            documents or instruments, in the form presented to Tenant, which
            Landlord, Lessor or Holder deems necessary or desirable to make this
            Lease prior and superior to the lease, mortgage or deed of trust.
            [SEE ADDENDUM]

      c.    RECOGNITION OR ATTORNMENT AGREEMENT. If Landlord or Holder requests
            Tenant to execute a document subordinating this Lease, the document
            shall provide that, so long as Tenant is not in default, Lessor or
            Holder shall agree to enter into either a recognition or attornment
            agreement with Tenant, or a new lease with Tenant upon the same
            terms and conditions as to possession of the Premises, which shall
            provide that Tenant may continue to occupy the Premises so long as
            Tenant shall pay the Rent and observe and perform all the provisions
            of this Lease to be observed and performed by Tenant. [SEE ADDENDUM]

24.   TRANSFER OF THE PROPERTY.

      Upon transfer of the Property and assignment of this Lease, Landlord shall
      be entirely freed and relieved of all liability under any and all of its
      covenants and obligations contained in or derived from this Lease
      occurring after the consummation of the transfer and assignment, and from
      all liability for the Security Deposit. Tenant shall attorn to any entity
      purchasing or otherwise acquiring the Premises at any sale or other
      proceeding.

25.   ESTOPPEL CERTIFICATES.

      Within ten (10) days following written request by Landlord, Tenant shall
      execute and deliver to Landlord an estoppel certificate, in the form
      prepared by Landlord. The certificate shall: (i) certify that this Lease
      is unmodified and in full force and effect or, if modified, state the
      nature of the modification and certify that this Lease, as so modified, is
      in full force and effect, and the date to which the Rent and other charges
      are paid in advance, if any; (ii) acknowledge that there are not, to
      Tenant's knowledge, any uncured defaults on the part of Landlord
      hereunder, or if there are uncured defaults on the part of the Landlord,
      state the nature of the uncured defaults; (iii) evidence the status of
      this Lease as may be required either by a lender making a loan to Landlord
      to be secured by deed of trust or mortgage covering the Premises or a
      purchaser of the Property from Landlord and (iv) such other information
      that is reasonably requested.

26.   MORTGAGEE PROTECTION.

      In the event of any default on the part of Landlord, Tenant will give
      notice by registered or certified mail to the address provided by Landlord
      to any beneficiary of a deed of trust or mortgagee of a mortgage covering
      the Property and shall offer the beneficiary or mortgagee a reasonable
      opportunity to cure the default, [Intentionally deleted].

27.   ATTORNEYS' FEES.

      If either party shall bring any action or legal proceeding for damages for
      an alleged breach of any provision of this Lease, to recover rent or other
      sums due, to terminate the tenancy of the Premises or to enforce, protect
      or establish any term, condition or covenant of this Lease or right of
      either party, the prevailing party shall be entitled to recover, as a part
      of the action or proceedings, or in a separate action brought for that
      purpose, reasonable attorneys' fees and court costs as may be fixed by the
      court or jury. [Intentionally deleted] [SEE ADDENDUM]

28.   BROKERS.

      Tenant warrants and represents that it has had no dealings with any real
      estate broker or agent in connection with the negotiation of this Lease,
      except for any brokers(s) specified in the Basic Lease Information, and
      that it knows of no other real estate broker or agent who is or might be
      entitled to a commission in connection with this Lease. Tenant shall
      indemnify and hold harmless Landlord from and against any and all
      liabilities or expenses arising out of claims made by any other broker or
      individual for commissions or fees resulting from this Lease.
      [SEE ADDENDUM]

29.   PARKING.

      Tenant shall have the right to park in the Building's parking facilities,
      in common with other tenants of the Building, upon such terms and
      conditions as may from time to time be established by Landlord. There
      shall be no charge for any portion of the parking facilities which is not
      reserved. Tenant agrees not to use in excess of its proportionate share of
      parking facilities and agrees to cooperate with Landlord and other tenants
      in the use of the parking facilities. Landlord reserves the right, in its
      absolute discretion, to determine whether the parking

                                       10
<PAGE>   14

      facilities are becoming crowded and to allocate and assign parking spaces
      among Tenant and the other tenants. Landlord shall not be liable to
      Tenant, nor shall this Lease be affected, if any parking is impaired by
      moratorium, initiative, referendum, law, ordinance, regulation or order
      passed, issued or made by any governmental or quasi-governmental body.
      Tenant shall not park its vehicles in any parking areas designated by
      Landlord as areas for parking by visitors to the Building or other
      reserved parking spaces. Tenant shall not park vehicles in the Building
      parking areas other than automobiles, motorcycles, motor driven or
      non-motor driven bicycles, or four-wheeled trucks. Tenant shall not leave
      vehicles in the Building parking areas overnight without Landlord's prior
      approval. Tenant and its agents, employees and invitees shall not park any
      one vehicle in more than one parking space.

30.   UTILITIES AND SERVICES.

      [intentionally deleted] Landlord agrees to furnish, or cause to be
      furnished, to the Premises the utilities and services described in the
      standards for Utilities and Services, set forth in Exhibit E, subject to
      the conditions and in accordance with the standards set forth therein.
      Landlord shall not be liable for, and Tenant shall not be entitled to any
      abatement or reduction of Rent by reason of, no eviction of Tenant shall
      result from and, further, Tenant shall not be relieved from the
      performance of any covenant or agreement in this Lease because of,
      Landlord's failure to furnish any of the foregoing when the failure is
      caused by accident, breakage, or repairs, strikes, lockouts or other labor
      disturbance or labor dispute of any character, governmental regulation,
      moratorium or other governmental action, inability despite the exercise of
      reasonable diligence to obtain electricity, water or fuel, or by any other
      cause beyond Landlord's reasonable control. In the event of any failure,
      stoppage or interruption thereof, Landlord shall diligently attempt to
      resume service. [SEE ADDENDUM]

31.   TENANT PLACEMENT.

      [intentionally deleted]

32.   ACCEPTANCE.

      Delivery of this Lease, duly executed by Tenant, constitutes an offer to
      lease the Premises as set forth herein, and under no circumstances shall
      such delivery be deemed to create an option or reservation to lease the
      Premises for the benefit of Tenant. This Lease shall become effective and
      binding only upon execution hereof by Landlord and delivery of a signed
      copy to Tenant. [intentionally deleted] [SEE ADDENDUM]

33.   USE OF BUILDING NAME.

      Tenant shall not employ the name of the Building in the name or title of
      its business or occupation without Landlord's prior written consent, which
      consent Landlord may withhold in its sole discretion. Landlord reserves
      the right to change the name of the Building without Tenant's consent and
      without any liability to Landlord.

34.   RECORDING.

      Neither Landlord nor Tenant shall record this Lease, nor a short form
      memorandum of this Lease, without the prior written consent of the other.

35.   QUITCLAIM.

      Upon any termination or expiration of this Lease pursuant to its terms,
      Tenant, at Landlord's request, shall execute, have acknowledged and
      deliver to Landlord a quitclaim deed of all Tenant's interest in the
      Premises, Building and Property created by this Lease.

36.   NOTICES.

      Any notice or demand required or desired to be given under this Lease
      shall be in writing and shall be given by hand delivery, facsimile or the
      United States mail. Notices which are sent by facsimile shall be deemed to
      have been given upon receipt. Notices which are mailed

                                       11
<PAGE>   15

      shall be deemed to have been given when seventy-two (72) hours have
      elapsed after the notice was deposited in the United States mail,
      registered or certified, the postage prepaid, addressed to the party to be
      served. As of the date of execution of this Lease, the addresses of
      Landlord and Tenant are as specified in the Basic Lease Information.
      Either party may change its address by giving notice of the change in
      accordance with this Section.

37.   LANDLORD'S EXCULPATION.

      In the event of default, breach or violation by Landlord (which term
      includes Landlord's partners, co-venturers and co-tenants, and officers,
      directors, members, employees, agents and representatives of Landlord and
      Landlord's partners, co-venturers and co-tenants) of any of Landlord's
      obligations under this Lease, Landlord's liability to Tenant shall be
      limited to its ownership interest in the Building and Property or the
      proceeds of a public sale of the ownership interest pursuant to the
      foreclosure of a judgment against Landlord. Landlord shall not be
      personally liable, or liable in any event, for any deficiency beyond its
      ownership interest in the Building and Property.

38.   ADDITIONAL STRUCTURES.

      Any diminution or interference with light, air or view by any structure
      which may be erected on land adjacent to the Building shall in no way
      alter this Lease or impose any liability on Landlord.

39.   GENERAL.

      a.    CAPTIONS. The captions and headings used in this Lease are for the
            purpose of convenience only and shall not be construed to limit or
            extend the meaning of any part of this Lease.

      b.    TIME. Time is of the essence for the performance of each term,
            condition and covenant of this Lease.

      c.    SEVERABILITY. If any provision of this Lease is held to be invalid,
            illegal or unenforceable, the invalidity, illegality, or
            unenforceability shall not affect any other provision of this Lease,
            but this Lease shall be construed as if the invalid, illegal or
            unenforceable provision had not been contained herein.

      d.    CHOICE OF LAW; CONSTRUCTION. This Lease shall be construed and
            enforced in accordance with the laws of the State of California. The
            language in all parts of this Lease shall in all cases be construed
            as a whole according to its fair meaning and not strictly for or
            against either Landlord or Tenant.

      e.    GENDER; SINGULAR, PLURAL. When the context of this Lease requires,
            the neuter gender includes the masculine, the feminine, a
            partnership or corporation or joint venture, and the singular
            includes the plural.

      f.    BINDING EFFECT. The covenants and agreements contained in this Lease
            shall be binding on the parties hereto and on their respective
            successors and assigns (to the extent this Lease is assignable).

      g.    WAIVER. The waiver of Landlord of any breach of any term, condition
            or covenant of this Lease shall not be deemed to be a waiver of the
            provision or any subsequent breach of the same or any other term,
            condition or covenant of this Lease. The subsequent acceptance of
            Rent hereunder by Landlord shall not be deemed to be a waiver of any
            preceding breach at the time of acceptance of the payment. No
            covenant, term or condition of this Lease shall be deemed to have
            been waived by Landlord unless the waiver is in writing signed by
            Landlord.

      h.    ENTIRE AGREEMENT. This Lease is the entire agreement between the
            parties, and there are no agreements or representations between the
            parties except as expressed herein. Except as otherwise provided
            herein, no subsequent change or addition to this Lease shall be
            binding unless in writing and signed by the parties hereto.

      i.    COUNTERPARTS. This Lease may be executed in counterparts, each of
            which shall be an original, but all counterparts shall constitute
            one (1) instrument.

      j.    EXHIBITS. The Basic Lease Information and all exhibits attached
            hereto are hereby incorporated herein and made an integral part
            hereof.

      k.    MULTIPLE ENTITIES COMPRISING TENANT. All persons and entities
            comprising Tenant, including all general partners (if any) of
            Tenant, are jointly and individually bound to perform each and every
            obligation of Tenant under this Lease and are jointly and
            individually liable to Landlord for Tenant's performance.

                                       12
<PAGE>   16

      l.    ADDENDUM. The Addendum, if any, attached hereto is hereby
            incorporated herein and made an integral part hereof.

      m.    WAIVER OF JURY TRIAL. Landlord and Tenant waive the right to trial
            by jury in any action, proceeding or counterclaim brought by either
            of the parties to this Lease against the other on any matters
            whatsoever arising out of or in any way connecting with this Lease,
            the relationship of Landlord and Tenant, or Tenant's use or
            occupancy of the Premises. [SEE ADDENDUM]

IN WITNESS WHEREOF, THE PARTIES HAVE EXECUTED THIS LEASE ON THE DATES SET FORTH
BELOW, EFFECTIVE AS OF THE DATE FIRST ABOVE WRITTEN.

                                         LANDLORD:
                                         STONY POINT EAST,
                                         A CALIFORNIA GENERAL PARTNERSHIP

DATE
EXECUTED: 4/26/99                        BY: /s/ JAMES BRECHT
         ---------------                    ------------------------------------
                                            NAME:
                                                 -------------------------------
                                            ITS:
                                                --------------------------------

                                         TENANT:
                                         ADVANCED TELCOM GROUP,
                                         A DELAWARE CORPORATION

DATE
EXECUTED: 4/26/99                        BY: /s/ C. G. RUDOLPH
         ---------------                    ------------------------------------
                                            NAME: C. G. Rudolph
                                                 -------------------------------
                                            ITS:  Chairman & CEO
                                                --------------------------------
DATE
EXECUTED:                                BY:
         ---------------                    ------------------------------------
                                            NAME:
                                                 -------------------------------
                                            ITS:
                                                --------------------------------

                                       13
<PAGE>   17

                                    EXHIBIT A

                                    SITE PLAN

                                     [MAP]

<PAGE>   18
                                                                       EXHIBIT B

                                  WORK LETTER

     This Work Letter states Landlord's and Tenant's responsibilities,
respectively, for the construction of the Building; and the Premises.

     1.   DEFINED TERMS. Unless provided to the contrary herein, the following
defined terms shall have the meanings set forth below and the remaining defined
terms shall have the meanings set forth in the Lease:

          Landlord's Representative:         James W. Brecht

          Tenant's Representative:
                                             --------------------------------

          Additional Allowance for
          Tenant Improvements:               [*]

     2.   LANDLORD'S WORK.

     2.1.  Building Shell/Site Work Interior Improvements and Tenant
Improvements. Landlord, at its sole cost and expense, shall arrange for the
construction by Landlord's contractor ("Landlord's Contractor") of (i) a
two-story building (the "Building Shell") consisting of approximately sixty-six
thousand eight hundred seventy-three (66,873) rentable square feet of space, in
accordance with the final working drawings and specifications (the "Building
Shell/Site Work Working Drawings), more particularly described in Exhibit B-1,
attached hereto and incorporated herein by reference; (ii) certain interior
improvements in the Building (the "Interior Improvements"), in accordance with
the final working drawings and specifications (the "Interior Improvements
Working Drawings"), listed on Exhibit B-2, attached hereto and incorporation
herein by reference, and (iii) certain tenant improvements in the Building (the
"Tenant Improvements") in accordance with the preliminary design (the "Tenant
Preliminary Design") and outline specifications (the "Tenant Improvement
Specifications"), each listed on Exhibit B-3, attached hereto and incorporated
herein by reference. The construction of the Building Shell/Site Work, the
Interior Improvements and the Tenant Improvements is hereinafter referred to as
"Landlord's Work."

     2.2.  Costs of Landlord's Work. Except as provided in Section 2.3 below,
Landlord shall pay for all fees incurred in connection with the construction of
Landlord's Work.

     2.3.  Tenant's Responsibility For Costs of Landlord's Work. Tenant shall
pay to Landlord, within ten (10) days after Landlord's written request therefor,
the positive difference between (i) Landlord's estimate of any additional costs
that Landlord anticipates it will incur in connection with the construction of
Landlord's Work as a result of any changes to Landlord's Work requested by
Tenant and approved by Landlord (which approval shall not be unreasonably

-----------------------
[*]  Information redacted pursuant to a confidential treatment request
     throughout this exhibit.
<PAGE>   19
withheld) and (ii) the amount of the Additional Allowance for Tenant
Improvements available to pay for such costs.

          2.4. Substantial Completion. The Building Shell/Site Work, the
Interior Improvements and the Tenant Improvements shall be deemed "Substantially
Complete" when (i) Landlord's Contractor has substantially completed the
Building Shell/Site Work, the Interior Improvements and the Tenant
Improvements, event though minor items may remain to be installed, finished or
corrected, provided such minor items do not have any material effect on the
ability of Tenant to utilize the Building Shell/Site Work, the Interior
Improvements and the Tenant Improvements for their intended purpose; and (ii)
Landlord delivers to Tenant a certificate from Landlord's architect ("Landlord's
Architect"), certifying (a) that the Building Shell/Site Work, the Interior
Improvements and the Tenant Improvements are "Substantially Complete" in
accordance with the terms of this Work Letter and (b) the date on which the
Building Shell/Site Work, the Interior Improvements and the Tenant Improvements
were Substantially Completed. Landlord shall use diligent efforts to obtain and
furnish Tenant with a "finaled" building permit with respect to the Building
Shell/Site Work, the Interior Improvements and the Tenant Improvements after
completing the Building Shell/Site Work, the Interior Improvements and the
Tenant Improvements. Substantial Completion shall have occurred notwithstanding
Tenant's submission of a punchlist to Landlord, which Tenant and Landlord shall
develop jointly within thirty (30) days after the Commencement Date. The
punchlist shall be based upon an inspection of the Premises by Landlord's
Representative and Tenant's Representative. Landlord shall commence and use
commercially reasonable efforts to complete any items of Landlord's Work not
completed as of the Commencement Date within thirty (30) days after the
Commencement Date.

          2.5.  Warranty.  Notwithstanding anything to the contrary in the
Lease, effective as of the date upon which Landlord delivers possession of the
Premises to Tenant, Landlord does hereby warrant that (a) the construction of
Landlord's Work was performed in accordance with all rules, regulations, codes,
statutes, ordinances, and laws of all governmental and quasi-governmental
authorities, in accordance with the Building Shell/Site Work Working Drawings
and the Tenant Improvement Specifications (collectively, the "Plans"), and in a
good and workman-like manner, (b) all material and equipment installed therein
conformed to the Plans and was new and otherwise of good quality, (c) the
electrical, plumbing, and mechanical systems servicing the Premises are in
working order and in good condition, and (d) the roof is in good condition and
water tight.

     3.   ADDITIONAL TENANT IMPROVEMENTS.

          3.1. Responsibility for Tenant Improvements.  Tenant shall be
responsible for all costs of any additional improvements to the Premises (the
"Additional Tenant Improvements"), including architectural, engineering,
consultant, utility, development, transaction and building permit and impact
fees (subject only to Landlord's obligation to provide the Additional Allowance
for Tenant Improvements and set forth below), and all trade fixtures, equipment,
furniture or other improvements installed by Tenant in the Premises. The
Additional Tenant Improvements shall be treated as alterations and shall be
subject to the terms of Section 9 of the Lease, as modified by this Work Letter.
The design, permitting and construction of the

                                       2
<PAGE>   20

Additional Tenant Improvements are hereinafter collectively referred to as the
"Additional Tenant Improvement Work."

     3.2.  Tenant's Architect and Contractor. If Tenant's architect is an
architect other than Simons & Brecht Inc., or Tenant's Contractor (as
hereinafter defined) is a contractor other than Landlord's Contractor, then,
prior to commencing the Additional Tenant Improvement Work, Tenant shall obtain
Landlord's prior written consent to Tenant's architect and contractor. Landlord
shall not unreasonably withhold its approval of any architect or contractor
selected by Tenant. Tenant's architect and contractor must each be licensed to
do business in California The architect and the contractor selected by Tenant
and approved by Landlord (as provided above) are hereinafter referred to as
"Tenant's Architect" and "Tenant's Contractor," respectively.

     3.3.  Changes. Tenant shall not make any material changes to the Building
Shell/Site Work Working Drawings, the Interior Improvements Working Drawings or
the Tenant Improvement Specifications described in Exhibit B-1, Exhibit B-2
and Exhibit B-3 respectively, without Landlord's prior written approval, which
approval shall not be unreasonably withheld or delayed. All material changes to
the Building Shell/Site Work Working Drawings, the Interior Improvements
Working Drawings or the Tenant Improvements Specifications must be in writing
and signed by both Landlord and Tenant prior to the change being made. Tenant
shall be responsible for all additional costs attributable to changes requested
by Tenant, including, without limitation, additional architectural fees and
increases in construction costs of Landlord's Work and the Additional Tenant
Improvements Work.

     3.4.  Notice. If Tenant's Contractor is not the same as Landlord's
Contractor, Tenant shall provide Landlord with not less than ten (10) days'
written notice prior to the commencement of any Additional Tenant Improvement
Work and permit Landlord to post on the Premises such notices of
nonresponsibility as may be required or otherwise available to Landlord.
Landlord, at its option, may require Tenant to provide to Landlord, at Tenant's
sole cost and expense, a lien and completion bond in an amount equal to the
estimated cost of the Additional Tenant Improvement Work (as reasonably
determined by Landlord) to insure Landlord against liability for mechanic's and
materialmen's liens and to insure completion of the Additional Tenant
Improvement Work.

     3.5.  Costs. Tenant shall promptly pay any and all costs and expenses in
connection with or arising out of the Additional Tenant Improvement Work and
shall furnish to Landlord evidence of such payment upon request. If the
Additional Improvement Work is to be performed by Tenant's Contractor, upon
completion of the Additional Tenant Improvement Work, Tenant shall deliver to
Landlord a release and waiver of lien executed by each contractor,
subcontractor and materialman concerned with the Additional Tenant Improvement
Work. In the event any lien is filed against the Premises or any portion
thereof or against Tenant's leasehold interest therein, Tenant shall record in
the Official Records of Sonoma County a release bond executed by an admitted
surety insurer authorized to issue surety bonds in the State of California with
respect to the lien or shall obtain the release and/or discharge of the lien
within seven (7) days after the filing thereof. In the event Tenant fails to do
so, Landlord may, after notice to Tenant, obtain release and/or discharge of the
lien and Tenant shall indemnify Landlord for the costs thereof, including
reasonable attorneys' fees, together with interest at the Interest Rate from
the date of demand.

                                       3
<PAGE>   21
          3.6.  Indemnity. Tenant shall indemnify, protect, defend (with counsel
satisfactory to Landlord) and hold harmless Landlord and its officers,
directors, employees, shareholders, partners and members from and against any
and all suits, claims, actions, loss, costs or expense (including claims for
workers' compensation, attorneys' fees and costs) based on personal injury or
property damage caused in, or contract claims (including, claims for breach of
warranty) arising from, the performance of the Additional Tenant Improvement
Work. Tenant shall repair or replace any portion of the Building or item of
Landlord's equipment or Landlord's Contractor's equipment or any of Landlord's
real or personal property damaged, lost or destroyed in the performance of the
Additional Tenant Improvement Work.

          3.7.  Insurance. Within fifteen (15) days after the approval of
Tenant's Contractor, Tenant and Tenant's Contractor shall obtain and provide
Landlord with certificates evidencing workers' compensation, public liability
and property damage insurance in amounts and forms and with companies reasonably
satisfactory to Landlord.

     4. COOPERATION. Tenant shall cooperate and diligently assist Landlord's
Architect and Landlord's Contractor in completing the preliminary plans and
working drawings and specifications for Landlord's Work. Tenant and Tenant's
Contractor shall not interfere with Landlord's construction of Landlord's Work.

     5. ADDITIONAL ALLOWANCE FOR TENANT IMPROVEMENTS.

          5.1.  Allowance. Landlord shall make available to Tenant upon the
terms and conditions set forth below an additional allowance for tenant
improvements (the "Additional Allowance for Tenant Improvements") in the amount
of [*]. Tenant may use the Additional Allowance for Tenant Improvements to pay
for Additional Tenant Improvement Allowance Items and any charges pursuant to
Section 2.3 of this Work Letter.

          5.2.  Additional Tenant Improvement Allowance Items. The term
"Additional Tenant Improvement Allowance Items" shall include work performed and
materials furnished in connection with the construction of the Additional Tenant
Improvements pursuant to this Work Letter and charges pursuant to Section 2.3 of
this Work Letter. If there is any Additional Allowance for Tenant Improvements
remaining after completion of, and payment of all amounts due in connection
with, all of the Additional Tenant Improvements and payment of all charges
pursuant to Section 2.3 of this Work Letter, Tenant shall have the right to use
the remaining amount of the Additional Allowance for Tenant Improvements for
trade fixtures, furniture, furnishings, equipment (except electrical, mechanical
and plumbing systems, including HVAC systems), or design, engineering,
architectural, utility and permit fees and/or the payment of rent under this
Lease.

     6. TENANT DELAYS.

          6.1.  Commencement Date. If Landlord fails to Substantially Complete
the Building Shell/Site Work, the Interior Improvements or the Tenant
Improvements on or before the Estimated Commencement Date, or thereafter, and if
the cause of the delay in Landlord delivering the Premises to Tenant by the
Estimated Commencement Date, or thereafter, is

-----------------------
[*]  Information redacted pursuant to a confidential treatment request
     throughout this exhibit.

                                       4
<PAGE>   22

attributable to Tenant, then the Commencement Date for all purposes under the
Lease will be the day on which the Building Shell/Site Work, the Interior
Improvements and the Tenant Improvements would have been Substantially Complete
absent such Tenant Delays (defined below).

            6.2.  Tenant Delays. Delays attributable to Tenant ("Tenant Delays")
shall include any interference with or delay in the completion of Landlord's
Work or the Additional Tenant Improvement Work (to the extent performed by
Landlord or Landlord's Contractor) caused by:

                  6.2.1. Tenant, Tenant's Architect, Tenant's Contractor or any
representative, employee, agent or subcontractor of any of the aforementioned
parties;

                  6.2.2. Tenant's early entry in the Premises or failure to
perform its obligations under this Lease;

                  6.2.3. Tenant's change requests that result in delays; or

                  6.2.4. Tenant's failure to provide approvals or disapprovals
in a timely manner, to the extent the approvals are necessary.

      7. REPRESENTATIVES.

            7.1. Tenant's Representative. Tenant has designated Tenant's
Representative as its sole representative with respect to the matters set forth
in this Work Letter, who shall have full authority and responsibility to act on
behalf of Tenant as required in this Work Letter. Tenant shall not change
Tenant's Representative without notice to Landlord.

            7.2. Landlord's Representative. Landlord has designated Landlord's
Representative as its sole representative with respect to the matters set forth
in this Work Letter, who shall have full authority and responsibility to act on
behalf of Landlord as required in this Work Letter. Landlord shall not change
Landlord's Representative without notice to Tenant.

      8. FORCE MAJEURE. Whenever a period of time or a specific date is
prescribed in this Work Letter or the Lease for action to be taken by Landlord,
Landlord shall not be liable or responsible for, and there shall be excluded
from the computation for any such period of time (or the specified date shall be
deferred by the number of days of) any delays in the obtaining of any permits
for and in the construction of the work to be performed by such party caused by
any action, claim, inaction, order, ruling, moratorium, regulation, statute,
condition or other decision of any governmental agency having jurisdiction over
any portion of the Property, over the construction anticipated to occur thereon
or over any uses thereof or by fire, flood, inclement weather (including rain),
strikes, lockouts or other labor or industrial disturbance, civil disturbance,
order of any government, court or regulatory body claiming jurisdiction or
otherwise, act of public enemy, war, riot, sabotage, blockage, embargo, failure
of inability to secure materials, supplies or labor through ordinary sources by
reason of shortages or priority or similar regulation, order of any government
or regulatory body, lightning, earthquake, storm, hurricane, tornado, washout,
explosion or any cause whatsoever beyond the reasonable control of the party
where performance is required, or any of its contractors or other
representatives,

                                       5

<PAGE>   23

whether or not similar to any of the causes hereinabove stated (each an "Event
of Force Majeure").

                                       6
<PAGE>   24

                                   EXHIBIT B-1

           BUILDING SHELL/SITE WORK WORKING DRAWINGS & SPECIFICATIONS

                 Attached hereto and made a part of this Lease.

<PAGE>   25

                                   EXHIBIT B-2

              LOBBY/CORRIDOR INTERIOR IMPROVEMENT WORKING DRAWINGS
                                       &
                                 SPECIFICATIONS

                 Attached hereto and made a part of this Lease.

<PAGE>   26

                                   EXHIBIT B-3

                            TENANT PRELIMINARY DESIGN
                                        &
                    OUTLINE TENANT IMPROVEMENT SPECIFICATIONS

                 Attached hereto and made a part of this Lease.

<PAGE>   27

                         TO BE COMPLETED UPON OCCUPANCY

                                    EXHIBIT C

                          COMMENCEMENT DATE MEMORANDUM

LANDLORD:
                            a
TENANT:
                            a
LEASE DATE:
PREMISES:
                         Santa Rosa, California  95401

Pursuant to Section 2.d. of the above-referenced Lease, the Commencement Date
hereby is established as ________________, and the Expiration Date hereby is
established as ________________.

                                    LANDLORD

                                        a

                                    By
                                      ------------------------------------------
                                      Name:
                                           -------------------------------------
                                      Its:
                                          --------------------------------------

                                    TENANT

                                        a

                                    By
                                      ------------------------------------------
                                      Name:
                                           -------------------------------------
                                      Its:
                                          --------------------------------------

<PAGE>   28

                                    EXHIBIT D
                              RULES AND REGULATIONS

1.  No sign, placard, picture, advertisement, name or notice shall be installed
    or displayed on any part of the outside or inside of the Building without
    the prior written consent of Landlord. Landlord shall have the right to
    remove, at Tenant's expense, any sign installed or displayed in violation of
    this rule. All approved signs or lettering on doors and walls shall be
    painted, affixed or inscribed at the expense of Tenant by a person chosen by
    Landlord.

2.  The directory of the Building will be provided exclusively for the display
    of the name and location of tenants, and Landlord reserves the right to
    exclude any other names therefrom. Tenant shall pay Landlord's standard
    charge for any changes by Tenant.

3.  Except as consented to in writing by Landlord or in accordance with Building
    standard improvements, no draperies, curtains, shades, screens or other
    devices shall be hung at or used in connection with any window or exterior
    door or doors on the Premises. No awning shall be permitted on any part of
    the Premises. Tenant shall not place anything against or near glass
    partitions or doors or windows which may appear unsightly from outside the
    Premises.

4.  Tenant shall not obstruct any sidewalks, halls, lobbies, passages, exits,
    entrances, elevators or stairways of the Building. No tenant and no employee
    or invitee of any tenant shall go upon the roof of the Building or make any
    roof or terrace penetrations. Tenant shall not allow anything to be placed
    on the outside terraces or balconies without the prior written consent of
    Landlord.

5.  No Tenant shall invite to the Premises, or permit the visit of, persons in
    such numbers or under such conditions as to interfere with the use and
    enjoyment of the Common Areas of the Building by other tenants.

6.  All cleaning and janitorial services for the Building shall be provided
    exclusively through Landlord, and, except with the written consent of
    Landlord, no person or persons other than those approved by the Landlord
    shall be employed by Tenant or permitted to enter the Building for the
    purpose of cleaning. Tenant shall not cause any unnecessary labor by
    carelessness or indifference to the good order and cleanliness of the
    Premises. Landlord shall not in any way be responsible for any loss of
    property on the Premises, however occurring , or for any damage to any
    Tenant's property by the janitor or any other employee or person.

7.  Landlord will furnish Tenant, free of charge, one (1) key to Tenant's suite
    entrance for each two hundred fifty (250) rentable square feet of the
    Premises. Landlord may make a reasonable charge for any additional keys and
    for having any locks changed. Tenant shall not make or have made additional
    keys without Landlord's prior written consent, and Tenant shall not alter
    any lock or install a new additional lock or bolt on any door of its
    Premises without Landlord's prior written consent. Tenant shall deliver to
    Landlord, upon the termination of its tenancy, the keys to all locks for
    doors on the Premises. If Tenant loses any keys furnished by Landlord,
    Tenant shall pay Landlord the cost of rekeying the Premises.

8.  If Tenant requires telegraphic, telephonic, burglar alarm or similar
    services, it shall first obtain, and comply with, Landlord's instructions
    for their installation.

9.  The elevators shall be available for use by all tenants in the Building,
    subject to reasonable scheduling as Landlord in its discretion shall deem
    appropriate. No equipment, materials, furniture, packages, supplies,
    merchandise or other property will be received in the Building or carried in
    the elevators except between the hours, in the manner and in the elevators
    as may be designated by the Landlord.

10. Tenant shall not place a load upon any floor of the Premises which exceeds
    the maximum load per square foot which the floor was designed to carry and
    which is allowed by law. Tenant's business machines and mechanical equipment
    which cause noise or vibration which may be transmitted to the structure of
    the Building or to any space therein, and which is objectionable to Landlord
    or to any tenants in the Building shall be placed and maintained by the
    Tenant, at Tenant's expense, on vibration eliminators or other devices
    sufficient to eliminate noise or vibration.

11. Tenant shall not use or keep in the Premises any toxic or hazardous
    materials or any kerosene, gasoline or inflammable or combustible fluid or
    material other than those limited quantities necessary for the operation or
    maintenance of office equipment. Tenant shall not use or permit to be used
    in the Premises any foul or noxious gas or substance, or permit or allow the
    Premises to be occupied or used in a manner offensive or objectionable to
    Landlord or other occupants of the Building by reason of noise, odors or
    vibrations. No animal, except seeing eye dogs when in the company of their
    masters, may be brought into or kept in the Building.

12. Tenant shall not use any method of heating or air-conditioning other than
    that supplied by Landlord, unless Tenant receives the prior written consent
    of Landlord.

                                        1
<PAGE>   29

13. Tenant shall cooperate fully with Landlord to assure the most effective
    operation of the Building's heating and air-conditioning and to comply with
    any governmental energy-saving rules, laws or regulations of which Tenant
    has actual notice. Tenant shall refrain from attempting to adjust controls
    other than room thermostats installed for Tenant's use.

14. All entrance doors to the Premises shall be left locked when the Premises
    are not in use, and all doors opening to public corridors shall be kept
    closed except for normal ingress and egress to and from the Premises.

15. Landlord reserves the right, exercisable without notice and without
    liability to Tenant, to change the name and street address of the Building.

16. Landlord reserves the right to exclude any person from the Building after
    normal business hours, unless that person is known to the person or employee
    in charge of the Building or is properly identified. Tenant shall be
    responsible for all employees, clients or visitors and shall be liable to
    Landlord for all acts of those persons. Landlord shall not be liable for
    damages for any error in admitting or excluding any person from the
    Building. Landlord reserves the right to prevent access to the Building by
    closing the doors or by other appropriate action in case of invasion, mob,
    riot, public excitement or other commotion.

17. Tenant shall close and lock the door of its Premises, shut off all water
    faucets or other water apparatus and turn off all lights and other equipment
    which is not required to be continuously run. Tenant shall be responsible
    for any damage or injuries sustained by other tenants or occupants of the
    Building or Landlord for noncompliance with this Rule.

18. The toilet rooms, toilets, urinals, wash bowls and other apparatus shall not
    be used for any purpose other than that for which they were constructed, and
    no foreign substance of any kind whatsoever shall be placed therein. The
    expense of any breakage, stoppage or damage resulting from any violation of
    this rule shall be borne by the tenant who, or whose employees or invitees,
    shall have caused it.

19. Tenant shall not install any radio or television antenna, loudspeaker or
    other device on the roof or exterior walls of the Building. Tenant shall not
    interfere with radio or television broadcasting or reception from or in the
    Building or elsewhere.

20. Tenant shall not cut or bore holes for wires in the partitions, woodwork or
    plaster of the Premises. Tenant shall not affix any floor covering to the
    floor of the Premises in any manner except as approved by Landlord. Landlord
    shall approve in writing the method of attachment of any objects affixed to
    walls, ceilings or doors in the Premises. Tenant shall repair, or be
    responsible for the cost of repair of, any damage resulting from
    noncompliance with this rule.

21. Tenant shall not install, maintain or operate upon the Premises any vending
    machine without the prior written consent of Landlord.

22. Canvassing, soliciting and distributing handbills or any other written
    material and peddling in the Building are prohibited, and each tenant shall
    cooperate to prevent these activities.

23. Landlord reserves the right to exclude or expel from the Building any person
    who, in the Landlord's judgment, is intoxicated or under the influence of
    liquor or drugs, or who is in violation of any of the Rules and Regulations
    of the Building.

24. Tenant shall store all its trash and garbage within its Premises. Tenant
    shall not place in any trash box or receptacle any material which cannot be
    disposed of in the ordinary and customary manner of trash and garbage
    disposal within the Building. All garbage and refuse disposal shall be made
    in accordance with directions issued from time to time by Landlord.

25. Use by Tenant of Underwriters' Laboratory approved equipment for brewing
    coffee, tea, hot chocolate and similar beverages and microwaving food shall
    be permitted, provided that the equipment and use is in accordance with all
    applicable federal, state, county and city laws, codes, ordinances, rules
    and regulations.

26. Tenant shall not use the name of the Building in connection with or in
    promoting or advertising the business of Tenant, except as Tenant's address,
    without the written consent of Landlord.

27. Tenant shall comply with all safety, fire protection and evacuation
    procedures and regulations established by Landlord or any governmental
    agency. Tenant shall be responsible for any increased insurance premiums
    attributable to Tenant's use of the Premises, Building or Property.

28. Tenant assumes any and all responsibility for protecting its Premises from
    theft and robbery, which responsibility includes keeping doors locked and
    other means of entry to the Premises closed.

29. Tenant shall not use the Premises, or suffer or permit anything to be done
    on, in or about the Premises, which may result in an increase to Landlord in
    the cost of insurance maintained by Landlord on the building and Common
    Areas.

                                       2
<PAGE>   30

30. Tenant's requests for assistance will be attended to only upon appropriate
    application to the office of the Building by an authorized individual.
    Employees of Landlord shall not perform any work or do anything outside of
    their regular duties unless under special instructions from Landlord, and no
    employee of Landlord will admit any person (Tenant or otherwise) to any
    office without specific instructions from Landlord.

31. Tenant shall not park its vehicles in any parking areas designated by
    Landlord as areas for parking by visitors to the Building or other reserved
    parking spaces. Tenant shall not leave vehicles in the Building parking
    areas other than automobiles, motorcycles, motor driven or non-motor driven
    bicycles or four-wheeled trucks. Tenant shall not leave vehicles in the
    Building parking areas overnight. Tenant, its agents, employees and invitees
    shall not park any one (1) vehicle in more than one (1) parking space.

32. The scheduling and manner of all Tenant move-ins and move-outs shall be
    subject to the discretion and approval of Landlord, and move-ins and
    move-outs shall take place only after 6:00 p.m. on weekdays, on weekends, or
    at other times as Landlord may designate. Landlord shall have the right to
    approve or disapprove the movers or moving company employed by Tenant, and
    Tenant shall cause the movers to use only the entry doors and elevators
    designated by the Landlord. If Tenant's movers damage the elevator or any
    other part of the Property, Tenant shall pay to Landlord the amount required
    to repair the damage.

33. No cooking shall be permitted on the Premises, except with a microwave oven,
    nor shall the Premises be used for the storage of merchandise, for washing
    clothes, for lodging or for any improper, objectionable or immoral purpose.

34. Landlord shall have the right to control and operate the public portions of
    the Building, and the public facilities, heating and air conditioning, as
    well as facilities furnished for the common use of the tenants, in such
    manner as it deems best for the benefit of the tenants generally.

35. Landlord may waive any one or more of these Rules and Regulations for the
    benefit of Tenant or any other tenant, but no waiver by Landlord shall be
    construed as a waiver of the Rules and Regulations in favor of Tenant or any
    other tenant, nor prevent Landlord from thereafter enforcing the Rules and
    Regulations against any or all of the tenants of the Building.

36. These Rules and Regulations are in addition to, and shall not be construed
    to in any way modify or amend, in whole or in part, the terms, covenants,
    agreements and conditions of any lease of premises in the building.

37. Landlord reserves the right to make other reasonable Rules and Regulations
    as, in its judgment, may from time to time be needed for safety and
    security, for care and cleanliness of the Building and for the preservations
    of good order therein. Tenant agrees to abide by all Rules and Regulations
    hereinabove stated and any additional rules and regulations which are
    adopted.

38. Tenant shall be responsible for the observance of all of the foregoing rules
    by Tenant's employees, agents, clients, customers, invitees and guests.

                                       3
<PAGE>   31

                                    EXHIBIT E
                             UTILITIES AND SERVICES

The standards set forth below for Utilities and Services are in effect. Landlord
reserves the right to adopt nondiscriminatory modifications and additions
hereto, which do not materially affect Tenant's rights. Landlord shall give
notice to Tenant, in accordance with provisions of this Lease, of material
modification and additions.

1.      PROVISION BY LANDLORD.

        As long as Tenant is not in default under any of the terms of this
        Lease, Landlord shall provide:

        a.      ELEVATOR. Where applicable, provide unattended automatic
                elevator facilities Monday through Friday, except holidays, from
                8:00 a.m. to 6:00 p.m., and have at least one elevator available
                at all other times.

        b.      VENTILATION. Ventilate the Premises and furnish air-conditioning
                or heating Monday through Friday, except holidays, from 8:00
                a.m. to 6:00 p.m. (and at other times for the additional charges
                described in Paragraph 2) to the extent required for the
                comfortable occupancy of the Premises, subject to governmental
                regulation. The air-conditioning system achieves maximum cooling
                when the window coverings and sliding glass doors are closed.
                Landlord shall not be responsible for room temperatures if
                Tenant does not keep all sliding glass doors in the Premises
                closed whenever the system is in operation. Tenant shall
                cooperate to the best of its ability at all times with Landlord
                and shall abide by all reasonable regulations and requirements
                which Landlord may prescribe for the proper functioning and
                protection of the air-conditioning system. Tenant shall not
                connect any apparatus, device, conduit or pipe to the Building's
                chilled and hot water air-conditioning supply lines. Tenant and
                Tenant's servants, employees, agents, visitors, licensees or
                contractors shall not enter at any time the mechanical
                installations or facilities of the Building, or adjust, tamper
                with, touch or otherwise in any manner affect the installations
                or facilities. If any installation of partitions, equipment or
                fixtures by Tenant necessitates the re-balancing of the climate
                control equipment in the Premises, the re-balancing shall be
                performed by Landlord at Tenant's expense.

        c.      ELECTRICITY. Subject to the provisions of Paragraph 2, furnish
                to the Premises electric current as required by the Building
                standard office lighting and fractional horsepower office
                business machines in the amount of approximately one and
                eight-tenths (1.8) watts per square foot. If Tenant's electrical
                installation or electrical consumption is in excess of the
                quantity described above, or extends beyond normal business
                hours, Tenant shall reimburse Landlord monthly for the measured
                consumption. Tenant shall not connect any apparatus or device
                with wires, conduits or pipes, or other means by which the
                services are supplied, for the purpose of using additional or
                unusual amounts of the services without the prior written
                consent of Landlord. At all times Tenant's use of electric
                current shall not exceed the capacity of the feeders to the
                Building or the risers or wiring installation, except as
                provided in working drawings approved by Landlord.

        d.      WATER. Make water available in public areas for drinking and
                lavatory purposes only.

        e.      JANITORIAL SERVICE. Provide building standard janitorial service
                to the Premises, provided the Premises are used exclusively as
                offices, and are kept reasonably in order by Tenant. Tenant
                shall pay to Landlord any cost incurred by Landlord for
                janitorial services in excess of those generally provided for
                other tenants in the Building.

2.      ADDITIONAL CHARGES.

        Landlord may impose a reasonable charge for any utilities and services,
        including air-conditioning, electric current, water and janitorial
        service, required to be provided by Landlord by reason of (i) any use of
        the Premises at any time other than between the hours of 8:00 a.m. and
        6:00 p.m. Monday through Friday, except holidays; (ii) any use beyond
        what Landlord agrees to furnish as described above; or (iii) special
        electrical, cooling and ventilating needs created in certain areas by
        hybrid telephone equipment, computers and other similar equipment or
        uses.

3.      RULES AND REGULATIONS.

        Tenant agrees to cooperate at all times with Landlord and to abide by
        all reasonable regulations and requirements which Landlord may prescribe
        for the use of the utilities and services. Any failure to pay any excess
        costs as described above with the next installment of Rent due after
        receipt of a statement for such services shall constitute a breach of
        the obligation to pay Rent under this Lease and shall entitle Landlord
        to the rights granted in this Lease for a breach.

                                        1
<PAGE>   32

4.      STOPPING OF SERVICE.

        Landlord reserves the right to stop services of the elevator, plumbing,
        ventilation, air-conditioning and electric systems when necessary by
        reason of accident or emergency, or for repairs, alterations or
        improvements, in the judgment of Landlord desirable or necessary to be
        made, until the repairs, alterations or improvements have been
        completed. Landlord shall have no responsibility or liability for
        failure to supply elevator facilities, plumbing, ventilating,
        air-conditioning or electric service when prevented by strike or
        accident or by any cause beyond Landlord's reasonable control, or by
        laws, rules, orders, ordinances, directions, regulations or requirements
        of any federal, state, county or municipal authority or failure of gas,
        oil or other suitable fuel supply or inability by exercise of reasonable
        diligence to obtain gas, oil or other suitable fuel. It is expressly
        understood and agreed that any covenants on Landlord's part to furnish
        any service pursuant to any of the terms, covenants, conditions,
        provisions or agreements of this Lease, or to perform any act or thing
        for the benefit of Tenant, shall not be deemed breached if Landlord is
        unable to furnish or perform the same by virtue of a strike or labor
        trouble or any other cause whatsoever beyond Landlord's reasonable
        control.

5.      NOTICE.

        To the extent practical, Landlord shall attempt to give Tenant notice of
        proposed shutdowns of services.

6.      TELECOMMUNICATIONS RISERS AND CABLING

        Landlord shall not be responsible for the installation , operation or
        maintenance of any telecommunication risers and cabling from the minimum
        point of entry to the Building and, except to the extent of the gross
        negligence or willful misconduct of Landlord and its agents or
        contractors, Landlord shall have no liability to Tenant if, for whatever
        reason, service to the telecommunication risers and/or cabling fails.

7.      AFTER HOURS UTILITIES

        After hours HVAC service is available at the following rate: $15.00 per
        unit per hour for an individual request (two hour minimum if not
        immediately following or preceding normal Building business hours); and
        $10.00 per hour if service is scheduled year round.

        Requests for after hours HVAC must be made during business hours no
        later than forty-eight (48) hours prior to start of additional service.
        Landlord shall use its best efforts to accommodate all requests for
        after hours HVAC whenever received.

8.      BUILDING HOLIDAYS

        New Years Day
        Memorial Day
        Independence Day
        Labor Day
        Thanksgiving Day
        Day After Thanksgiving
        Christmas Day

        On the above holidays, the Building is closed but Tenant keys will
        access the Premises. All utilities except HVAC (unless requested by
        Tenant) are provided. Janitorial service is not provided.

        When a holiday falls on a Sunday, the Building will normally observe the
        holiday the following Monday. When a holiday falls on a Saturday, the
        Building will normally observe the holiday the preceding Friday.

                                       2
<PAGE>   33
                                   EXHIBIT F

                     [HARDING LAWSON ASSOCIATES LETTERHEAD]

September 15, 1988

1872,002.02

Stony Point West
709 Second Street
Santa Rosa, California 95404

Attention: Mr. Albert Kovanis

Gentlemen:

HAZARDOUS MATERIALS SITE ASSESSMENT
STONEY POINT ROAD PARCELS
SANTA ROSA, CALIFORNIA

This letter presents the results of a hazardous materials site assessment
conducted by Harding Lawson Associates (HLA) for an approximate 20-acre site
located at the intersection of West 9th Street and Stony Point Road in Santa
Rosa, California (Plate 1). The purpose of the assessment was to evaluate
whether past or present generation, use, storage or disposal of hazardous
materials occurred on the site and/or adjacent properties and whether such
materials could impact site development and/or require mitigation.

SCOPE OF SERVICES

To evaluate the potential impact of hazardous materials on the site, HLA's
investigation consisted of the following:

-    Reviewing documents and aerial photographs related to historical
     development of the site and adjacent property.

-    Contacting appropriate regulatory agencies for hazardous materials
     information concerning the site and the surrounding area located
     approximately within one-quarter mile of site boundaries; inquiries were
     made regarding documentation of toxic spills, underground tanks, the use,
     storage, disposal and/or generation of hazardous materials, and violations
     of applicable environmental control standards.

-    Reviewing selected reports for information pertaining to soil and
     ground-water contamination.

<PAGE>   34
September 15, 1988
1872,002.02
Mr. Albert Kovanis
Stony Point West
Page 2

-    Conducting an on-site inspection and off-site reconnaissance to identify
     visible evidence of hazardous materials generation, use, storage, spills
     or disposal.

-    Evaluating findings and preparing this report.

SITE DESCRIPTION

The site consists of four adjacent parcels of land near the intersection of
West 9th Street and Stony Point Road. The southern most parcel, AP Number
10-320-22, is a triangular piece of property bounded by West 9th Street to the
northwest, Stoney Point Road to the southwest, and existing residential
developments to the east. The southern tip of this property is adjacent to
Santa Rosa Creek. The remaining three parcels, A.P. Numbers 10-320-25, 26, and
27 comprise on continuous, irregularly shaped property, which is bounded by West
9th Street to the southwest, Stony Point Road to the southwest, undeveloped City
of Santa Rosa owned property to the northwest, and residential developments to
the north and east. All parcels are currently undeveloped and covered with
dried, volunteer grasses. The parcels are relatively flat, with the exception of
an approximately 5 foot high beam constructed adjacent to Stony Point Road and
West 9th Street on the irregularly shaped property.

SITE HISTORY

The study area, which includes the site and the surrounding areas, were used
for agricultural endeavor until the 1960s. Agriculture activity included the
growing of annual crops and fruit orchards. Since the end of agricultural
activities in the mid-1960s, the site has remained vacant and unused.
Development of the surrounding areas began in the 1950s with the construction of
Santa Rosa's wastewater treatment facility on West College Avenue. Two storage
ponds, which store an approximate 55 acre feet of treated wastewater, were built
at this time. Further development in the surrounding areas occurred during the
late 1960s and early 1970s. During this time, Stony Point Road, which previously
ended south of Santa Rosa Creek, was continued to the intersection of West
College Avenue and Marlow Road, approximately 1/4-mile northwest of the site.
Construction of residential subdivisions east of the property commenced during
this time period and continued through the 1970s. Stony Point West Business
Park, located west of the site, was constructed in the early 1980s.

AERIAL PHOTOGRAPH REVIEW

Historical and current land uses of the site and adjacent areas were reviewed
by examining aerial photographs at the Sonoma County Assessor's Office and at
Pacific Aerial Surveys in Oakland. Aerial photographs were taken in 1961, 1971,
1980, and 1986.

<PAGE>   35

September 15, 1988
1872,002.02
Mr. Albert Kovanis
Stony Point West
Page 3

In the May 1961 photograph, the site and surrounding areas are either orchards
or plowed. The Santa Rosa Waste Water Plant is the only major development in
the area.

Photographs in May 1971 show Stony Point Road completed along the site's
southwest property line. Agricultural activities at the site and adjacent areas
appear to have ended by this time. Initial grading for the residential
development east of the site is also shown. Photographs taken in July 1980 and
April 1986 show continued development of the surrounding residential areas and
construction of Stony Point Business Park on the southwest side of Stony Point
Road.

ON-SITE INSPECTION

An on-site inspection was performed by an HLA representative on August 17,
1988, to identify visual signs of potential liability related to improper
dumping of hazardous materials, surface impoundments, abandoned drums, or the
presence of aboveground or underground storage tanks. No visual signs of
potential contamination of any consequence were noted.

Small quantities of landscaping debris, consisting mainly of lawn cuttings,
were present on the northern property. Additionally, stockpiles of soil were
also present in the same area. Conversations with Mr. James Brecht indicate
that this soil was placed during construction the Stony Point Business Park,
southwest of Stony Point Road.

OFF-SITE RECONNAISSANCE

Reconnaissance of the area within 1/4-mile of the site was performed on August
17, 1988. No sources of hazardous materials in the form of storage tanks,
spills, or manufacturing processes that use hazardous materials were
identified. No businesses that might generate significant quantities of toxic
or hazardous materials are within 1/4-mile of the site.

AGENCY DISCUSSIONS

The following agency representatives were contacted regarding their knowledge
of potential or known environmental concerns at the site or surrounding
properties.

-     Janet Naito
      California Department of Health Services (DOHS), Emeryville
      Phone No.: 415/540-2043        Date: August 25, 1988

-     Luis Rivera
      Regional Water Quality Control Board (RWQCB), North Coast Division
      Phone No.: 707/576-2220        Date: August 25, 1988

<PAGE>   36

September 15, 1988
1872,002.02
Mr. Albert Kovanis
Stony Point West
Page 5

Conversations with personnel from the RWQCB and SRU indicate that the
wastewater ponds west of the site do not contain hazardous materials. These
ponds contain treated sewage and serve as storage ponds for irrigation water.
Treatment of wastewater at this plant occurs only during times of heavy
rainfall. Such events create wastewater in-flows which exceed the capacity of
the main treatment plant on Liano Road and require the use of the west college
facility for wastewater treatment.

CONCLUSIONS

Based on review of agency records, conversations with agency personnel, review
of in-house documents, and the site visit, there is no indication of obvious
hazardous or industrial waste contamination which may impact site development.

We trust that this is the information you require at this time. If you have any
questions, please call.

Yours very truly,

HARDING LAWSON ASSOCIATES

/s/ MICHAEL D. THOMPSON
-----------------------------
Michael D. Thompson
Project Engineer

/s/ WILLIAM F. FRIZZELL
-----------------------------
William F. Frizzell, P.E.
Manager, Site Assessment and Mitigation

TFW/WLF/ljc/B5094-CT

<PAGE>   37

                                   [AREA MAP]

[HARDING LAWSON ASSOCIATES LOGO]     Area Map                            PLATE 1
                                     Hazardous Waste Site Assessment
                                     Stony Point Road Parcels
                                     Santa Rosa, California

<PAGE>   38
                                   EXHIBIT G

                    LANDLORD WAIVER AND ESTOPPEL CERTIFICATE

First Union National Bank,
  as Administrative Agent
One First Union Center, TW-10
301 South College Street
Charlotte, NC 38288-0608

Ladies and Gentlemen:

     This letter acknowledged that the undersigned (the "Landlord") have been
informed that _________________ (the "Tenant"), has entered into a Credit
Agreement dated as of ____________, 1999 (as amended, modified, restated or
replaced from time to time, the "Credit Agreement") among Advanced Telecom
Group, Inc. and certain of its Subsidiaries (collectively the "Borrowers"),
First Union National Bank, as administrative agent (the "Administrative
Agent"), J.P. Morgan Bank & Trust as syndication agent, General Electric
Capital Corporation, as documentation agent and the lenders party thereto (the
"Lenders"). As a precedent condition to the Lender's obligation to provide
loans and other financial accommodations to the Tenant under the Credit
Agreement (the "Loans"), the Lenders require, among other things, liens on the
Tenant's personal property, including, without limitation equipment, and trade
fixtures (the "Personal Property").

     To induce the Lenders to continue to make the Loans and provide financial
accommodations to the Tenant, and for other good and valuable considerations,
the receipt and sufficiency of which are hereby acknowledged, the Landlord
hereby represents and warrants to the Administrative Agent that as of the date
hereof:

     1.   The Landlord is the current landlord under the lease described in
Exhibit A attached hereto (the "Lease") with the Tenant;

     2.   The Lease is in full force and effect and there have been no
amendments, modifications or other agreements relating to the Lease except as
stated herein;

     3.   To Landlord's actual knowledge, Tenant is not in default under the
Lease, nor are there in existence any facts which with notice or passage of
time or both would constitute a default or a breach by Tenant of the terms and
conditions of the Lease; and

     4.   The Tenant is the tenant under the Lease, and, to the Landlord
actual knowledge, is in full compliance with the covenants, agreements and
obligations under the Lease.

     The Landlord agrees that: (a) the Tenant (or its representatives) is the
owner of the Personal Property located at or on the Premises (as defined in the
Lease) whether attached to the Premises or not; (b) the Administrative Agent's
lien upon or security interest in the Personal Property is prior and superior
to any interest, lien or claim of any nature the Landlord may now have or
hereafter

<PAGE>   39
obtain in the Personal Property whether by operation of law, contract or
otherwise; (c) either the Tenant or the Administrative Agent may remove the
Personal Property from the Premises without hindrance on the part of the
Landlord subject to the conditions set forth herein. Upon not less than ten (10)
days' written request, the Landlord will grant the Administrative Agent (or its
representatives) access to the Premises so that the Administrative Agent (or its
representatives) may remove the Personal Property, provided that (1) the
Administrative Agent, on its own behalf and on behalf of the Lenders,
indemnifies and holds Landlord harmless from any claims, losses, liability,
costs or damages (including reasonable attorneys' fees) incurred by Landlord in
connection with the removal of the Personal Property by the Administrative Agent
and the Administrative Agent's entry on the Premises or Landlord's property, (2)
the Administrative Agent shall restore the Premises to its condition existing
immediately prior to the installation of the Personal Property removed by the
Administrative Agent, (3) in no event shall the Administrative Agent commit, or
permit to be committed, any damage to the Premises or any improvement of which
the Premises are a part, and (4) the Administrative Agent shall not conduct any
sales or auctions on the Premises; and (d) the Personal Property shall remain
personal property (to the extent such Personal Property is not already a fixture
as of the date hereof) and shall not become fixtures, notwithstanding the manner
or mode of the attachment of the Personal Property to the land. Landlord may
require that the Administrative Agent be accompanied by a representative of
Landlord when entering the Premises. The Landlord hereby waives any rights it
may now or hereafter have in the Personal Property, including without
limitation, any lien rights available under applicable law.

     The Landlord understands that the Administrative Agent and the Lenders
will rely on this Landlord Waiver and Estoppel Certificate in continuing to make
Loans and provide financial accommodations to the Tenant.

     Executed and delivered this ___ day of ____________, 1999.

                                        LANDLORD:

                                        Name: ________________________________

                                        By:   ________________________________

                                      -2-
<PAGE>   40

                               ADDENDUM TO LEASE
                                    Suite 200
                                 110 Stony Point
                             Santa Rosa, California

1.      Page 1 - Section 1:

        Line 4, after the word "restroom's;", insert "lobbies;".

2.      Page 1 - Section 2.b:

        Line 2, in place of the deleted language, insert "(i) the date on which
(a) Landlord has substantially completed Landlord's Work in accordance with the
terms of this Lease (except for punchlist items), (b) Landlord has delivered
possession of the Premises to Tenant and (c) Landlord has obtained a final
signed-off building permit or certificate of occupancy with respect to
Landlord's Work, (ii) the date Tenant occupies the Premises or (iii) the date
the Commencement Date would have occurred but for Tenant Delays, as described in
Exhibit B. In the event the Commencement Date shall not have occurred by March
15, 2000 for any reason whatsoever other than an Event of Force Majeure or a
Tenant Delay (each as defined in Exhibit B), then the date Tenant is otherwise
obligated to commence payment of rent shall be delayed by one additional day for
each day that the Commencement Date is delayed beyond March 15, 2000. In
addition, if the Commencement Date shall not have occurred by June 15, 2000 for
any for any reason whatsoever other than an Event of Force Majeure or a Tenant
Delay, then Tenant shall have the right to terminate this Lease by written
notice to Landlord prior to June 31, 2000, whereupon any monies previously paid
by Tenant to Landlord shall be reimbursed to Tenant. If Tenant fails to
terminate this Lease pursuant to the foregoing sentence by written notice to
Landlord prior to June 31, 2000, Tenant's right to terminate this Lease pursuant
to this Section 2.b shall terminate and be of no further force or effect.

3.      Page 1 - Section 2.d:

        At the end of this Section, insert "Tenant's failure to execute and
deliver to Landlord the Commencement Date Memorandum within five (5) days after
Tenant's receipt of the Commencement Date Memorandum shall be conclusive upon
Tenant as to the matters set forth in the Commencement Date Memorandum."

4.      Page 1 - Section 3.a:

        Line 10, in place of the deleted percentage, insert [*].

5.      Page 2 - Section 3.c:

        Add the following Section:

-----------------------
[*]  Information redacted pursuant to a confidential treatment request
     throughout this exhibit.
<PAGE>   41

        "c. Condition of Building. Landlord warrants and represents to Tenant
        that, as of the Commencement Date, (i) the Premises and the Building
        will comply with all applicable laws, rules, regulations, codes,
        ordinances, underwriters' requirements, covenants, conditions and
        restrictions of record (collectively, "Laws"), (ii) the Premises will be
        in good and clean operating condition and repair, (iii) the electrical,
        mechanical, HVAC, plumbing, sewer, elevator and other systems serving
        the Premises and the Building will be in good operating condition and
        repair, and (iv) the roof of the Building will be in good condition and
        water tight. Landlord shall, promptly after receipt of notice from
        Tenant, remedy any non-compliance with such warranty at Landlord's sole
        cost and expense."

6.      Page 2 - Section 4.a:

        Line 3, after the word "designate", insert ", without any prior demand
therefor and without any deduction, abatement or setoff except as specifically
provided in this Lease."

7.      Page 2 - Section 4.b:

        The following is inserted as Sections 4.b and c:

        "b. Increases In Base Rent. The Base Rent during the Term shall be as
        follows:

<TABLE>
<CAPTION>
Period                             Monthly Base Rent /RSF      Annual Base Rent / RSF
------                             ----------------------      ----------------------
<S>                                <C>                         <C>
Commencement Date through the
end of the third (3rd) year of
the Term                                       [*]              [*]
Fourth (4th) year of the Term                  [*]              [*]
Fifth (5th) year of the Term                   [*]              [*]
Sixth (6th) year of the Term                   [*]              [*]
Seventh (7th) year of the Term                 [*]              [*]
Eighth (8th) year of the Term                  [*]              [*]
Ninth (9th) year of the Term                   [*]              [*]
Tenth (10th) year of the Term                  [*]              [*]
</TABLE>

-----------------------
[*]  Information redacted pursuant to a confidential treatment request
     throughout this exhibit.

                                       2
<PAGE>   42

        c. Rent Abatement. Provided that no Event of Default has occurred which
        is continuing, Landlord agrees to waive a portion of the monthly Base
        Rent due under this Lease in the amount and during the period listed
        below:

<TABLE>
<CAPTION>
          Amount of Monthly Base Rent Waived      Period
          ----------------------------------      ------
<S>                                               <C>
                               [*]                Commencement Date through the last day of
                                                  the sixth (6th) month of the Term
                               [*]                The first day of the seventh (7th) month of
                                                  the Term through the last day of the ninth
                                                  (9th) month of the Term
                               [*]                The first day of the tenth (10th) month of
                                                  the Term through the last day of the twelfth
                                                  (12th) month of the Term
                               [*]                The first day of the thirteenth (13th) month
                                                  of the Term through the last day of the
                                                  fifteenth (15th) month of the Term
</TABLE>

8.      Page 2 - Section 5.b:

        Line 10, after the word "replacements", insert ", uninsured damage or
deductibles on property insurance,".

        Line 13, in place of the deleted language, insert "(x) the cost of any
capital improvements made to the Building or the Property, amortized over such
reasonable period as Landlord shall determine, together with interest upon the
unamortized balance at ten percent (10%) or such other higher rate as may have
been paid by Landlord on funds borrowed for the purpose of constructing the
capital improvements".

        At the end of this Section, insert "Notwithstanding anything to the
contrary contained in this Lease, Operating Expenses shall not include the
following: (a) any costs or expenses occasioned by Landlord's gross negligence
or willful misconduct or due to Landlord's violation of any Laws in effect as of
the Commencement Date; (b) any costs or expenses incurred to correct any
construction defect in the Premises or the Building or to comply with any
covenants, conditions or restrictions or Laws applicable to the Premises or the
Building on the Commencement Date; (c) costs or expenses incurred in renovating,
improving, painting or redecorating any other space in the Building for lease to
other tenants; (d) costs or expenses incurred in connection with marketing or
advertising the Building for sale or lease; (e) any costs or expenses incurred
as a result of the violation by Landlord or any other occupant in the Building
(other than Tenant or any assignee or sublessee of Tenant) of the terms and
conditions of any lease or other agreement; (f) insurance deductibles to the
extent that Tenant's share exceeds Twenty Thousand Dollars ($20,000) for any
single event; (g) costs or expenses incurred to remove or remediate any
Hazardous Substances (defined in Section 8.d) in the Building or on or under the
Property, except to the extent the Hazardous Substances were brought on or
released on the Property by or are attributable to the acts of Tenant or
Tenant's employees, agents, representatives, contractors, assignees or
subtenants; (h) costs or expenses in the nature of depreciation or other cash
reserves; (i) costs or expenses incurred to repair or restore the Premises or
the Building due to an uninsured casualty to the extent Tenant's share exceeds

-----------------------
[*]  Information redacted pursuant to a confidential treatment request
     throughout this exhibit.

                                       3
<PAGE>   43

Twenty Thousand Dollars ($20,000.00); (j) costs for capital improvements except
as provided in (x) above; and (k) management fees in excess of the management
fees charged by landlords of comparable office buildings in the same vicinity as
the Building. In addition, if Landlord does not carry earthquake or floor
insurance during the Base Year and Landlord later obtains earthquake or flood
insurance, then the amount of Operating Expenses paid or incurred by Landlord
for the Base Year shall be adjusted to an amount equal to the amount of
Operating Expenses that Landlord would have paid or incurred during the Base
Year if Landlord had obtained such earthquake or flood insurance during the Base
Year as reasonable determined by Landlord."

9.      Page 3 - Section 5.c:

        Line 4, after the word "estimate", insert "; provided however, if
Landlord fails to notify Tenant of the estimated amount of Tenant's share of
Operating Expenses for the ensuing accounting year prior to the end of the
current accounting year, Tenant shall be required to continue to pay to Landlord
each month in advance Tenant's estimated share of Operating Expenses on the
basis of the amount due for the immediately prior month until ten (10) days
after Landlord notifies Tenant of the estimated amount of Tenant's share of
Operating Expenses for the ensuing accounting year. If at any time it appears to
Landlord that Tenant's share of Operating Expenses payable for the current
accounting year will vary from Landlord's estimate, Landlord may give notice to
Tenant of Landlord's revised estimate for the accounting year, and subsequent
payments by Tenant for the accounting year shall be based on the revised
estimate."

        Line 5, in place of the deleted language, insert "detailing actual
Operating Expenses for the previous year".

        Line 7, after the word "Expenses", insert "in any year other than the
Base Year".

        Line 13, after the word "Expenses", insert "(including Operating
Expenses during the Base Year".

        At the end of this Section, insert "Tenant shall have ninety (90) days
after Tenant receives the year end statement of the Operating Expenses for the
prior calendar year to notify Landlord in writing of Tenant's desire to conduct,
at Tenant's sole cost and expense, an audit of Landlord's books and records
relating to the applicable calendar year. Any such audit must be conducted by
Tenant or its agent during regular business hours at the offices of Landlord or
the offices of Landlord's designated agent and must be completed within sixty
(60) days after Tenant notifies Landlord in writing of its election to audit the
applicable year end statement. The person or entity performing the audit or
review of Landlord's books and records on Tenant's behalf or at Tenant's request
may not be compensated for the audit or review on a contingency basis. If
Landlord objects to the findings of Tenant's audit, Landlord and Tenant shall
attempt to resolve their disagreement concerning the amount of Tenant's
proportionate share of Operating Expenses within the next thirty (30) days. If
Landlord and Tenant are unable to agree upon the amount of Tenant's
proportionate share of Operating Expenses (after Tenant has completed its
audit), the parties shall submit the matter to binding arbitration before a
single neutral arbitrator having experience in real estate valuation, property
management or accounting or, alternatively,

                                       4
<PAGE>   44

the arbitrator may be a retired judge or justice of a California Superior Court
or Court of Appeal. The matter shall be decided by arbitration in accordance
with the applicable arbitration statutes and the then existing Commercial
Arbitration Rules of the American Arbitration Association. Any party may
initiate the arbitration procedure by delivering a written notice of demand for
arbitration to the other party. Within thirty (30) days after the other party's
receipt of the written notice of demand for arbitration, the parties shall
attempt to select a qualified arbitrator who is acceptable to all parties. If
the parties are unable to agree upon an arbitrator who is acceptable to all
parties, either party may request the American Arbitration Association to
appoint the arbitrator in accordance with its Commercial Arbitration Rules. The
provisions of California Code of Civil Procedure Section 1283.05 or its
successor section(s) are incorporated in and made a part of this Lease with
respect to any arbitration requested in accordance with the provisions contained
in this Section. Depositions may be taken and discovery may be obtained in any
arbitration proceeding requested pursuant to this Section in accordance with the
provisions of California Code of Civil Procedure Section 1283.05 or its
successor section(s). Arbitration hearing(s) shall be conducted in Sonoma
County, California. Any relevant evidence, including hearsay, shall be admitted
by the arbitrator if it is the sort of evidence upon which responsible persons
are accustomed to rely in the conduct of serious affairs, regardless of the
admissibility of such evidence in a court of law; however, the arbitrator shall
apply California law relating to privileges and work product. In rendering his
or her award, the arbitrator shall set forth the reasons for his or her
decision. The fees and expenses of the arbitrator shall be paid in the manner
allocated by the arbitrator. This agreement to arbitrate any dispute concerning
the findings of Tenant's audit shall be specifically enforceable under the
prevailing arbitration law. Judgment on the award rendered by the award may be
entered in any court having jurisdiction thereof. If, as a result of Tenant's
audit, the parties determine that Landlord has overstated Tenant's percentage
share of the Operating Expenses by more than five percent (5%) during the
applicable calendar year, Landlord shall reimburse Tenant for the reasonable
cost of the audit."

10.     Page 3 - Section 5.d:

        At the end of this Section, insert "Notwithstanding anything to the
contrary contained in this Section, to the extent Real Property Taxes for the
Base Year are not based on a full-improved, fully-assessed Building, then, at
the time the Building is fully-improved and fully-assessed, the Real Property
Taxes for the Base Year shall be adjusted to reflect a fully-improved,
fully-assessed Building."

11.     Page 3 - Section 5.e:

        Line 3, after the word "Premises", insert ", Building or Property and
paid for by Tenant".

12.     Page 3 - Section 7:

        In place of the deleted language, insert "Landlord agrees to perform
certain work and construct certain tenant improvements in the Premises
(collectively, "Landlord's Work") pursuant to the terms of Exhibit B".

                                       5
<PAGE>   45

13.     Page 3 - Section 8.b:

        Line 2, after "Exhibit D", insert "and all reasonable amendments or
modifications thereto".

        Line 2, in place of the deleted word, insert "all present and future".

14.     Page 4 - Section 8.c:

        At the end of this Section, insert "Notwithstanding anything to the
contrary contained in this Lease, Tenant shall not be required to make any
capital improvements to the Premises in order to comply with or cause the
Premises to comply with any Laws unless such compliance is necessitated as a
result of Tenant's particular use of the Premises."

15.     Page 4 - Section 8.d:

        Add the following subsections:

             "(iv) Except as disclose in the documents listed on Exhibit F,
        attached hereto, to Landlord's actual knowledge, (a) there are no
        Hazardous Substances in the Building or on or under the Property in
        violation of any environmental Law; (b) there are no underground storage
        tanks under the Property; and (c) no action, proceeding or claim is
        pending or threatened regarding the Building or the Property concerning
        any Hazardous Substances or pursuant to any environmental Law.

             (v) Notwithstanding anything to the contrary contained in this
        Lease, Tenant shall have no obligation to pay for or to remediate any
        Hazardous Substances on, in or under the Premises or the Building,
        except to the extent the Hazardous Substances were brought or released
        on the Property or the Premises by Tenant or Tenant's agents,
        representatives, contractors, invitees, subtenants, successors or
        assigns.

16.     Page 4 - Section 9.a:

        At the end of this Section, insert "Tenant shall deliver to Landlord
full and complete plans and specifications of all alterations, and no work shall
be commenced by Tenant until Landlord has given its written approval of the
plans and specifications. Landlord does not expressly or implicitly covenant or
warrant that any plans or specifications submitted by Tenant are safe or comply
with all applicable laws and ordinances. Further, Tenant shall indemnify and
hold harmless Landlord from any loss, cost or expense, including attorneys' fees
and costs, incurred by Landlord as a result of any defects in design, materials
or workmanship resulting from Tenant's alterations to the Premises. All
alterations performed by Tenant shall be done in a good and workmanlike manner,
incorporating materials of quality equal to or better than those

                                       6
<PAGE>   46

replaced, with finishes comparable to and compatible with adjacent finishes
within the Premises and the Building. In addition, all of Tenant's alterations
shall be constructed in such a manner so as to (i) not unreasonably disturb or
otherwise interfere with the use and occupancy of any other tenant of the
Building, (ii) protect by appropriate means and measures all components of the
Premises and the Building from soiling or damage associated with Tenant's work,
and (iii) not impose any additional expense or delay upon Landlord in the
construction of improvements to, or maintenance or operation of, the Building or
the Property. Tenant shall reimburse Landlord for Landlord's reasonable charges
for reviewing and approving or disapproving plans and specifications for any
alterations proposed by Tenant (which reasonable charges may include the cost of
architectural services provided by Landlord)."

17.     Page 4 - Section 9.b:

        Line 4, in place of the deleted number, insert "ten (10)".

        Line 4, after the word "date", insert "on which Tenant is notified or
        becomes aware".

        At the end of this Section, insert "If Tenant fails to have the lien
discharged within the ten (10) day period referenced above, then Landlord may,
but shall not be required to, take such action or pay such amount as may be
necessary to remove such lien; and, Tenant shall pay to Landlord any amounts
expended by Landlord within five (5) days after Tenant receives Landlord's
written request for payment."

18.     Page 4 - Section 9.c:

        At the end of this Section, insert "Notwithstanding the foregoing, all
trade fixtures, furniture, equipment and other personal property installed or
placed in the Premises by Tenant ("Tenant's Property") shall at all times be and
remain Tenant's property. Tenant may at any time during the term of this Lease
remove Tenant's Property from the Premises, provided that Tenant repairs all
damage caused by such removal and restores the Premises to its condition
existing immediately prior to the installation of Tenant's Property in the
Premises."

19.     Page 5 - Section 10:

        At the end of this Section, insert "Landlord shall repair and maintain
the structural portions of the Premises and the Building (including the roof and
foundation), the Common Areas, and the Building's heating, ventilating, air
conditioning, electrical, water, sewer and plumbing systems; provided, however,
Landlord shall not be required to repair and maintain the electrical, sewer and
plumbing systems in the Premises."

20.     Page 5 - Section 11.b:

        Line 6, in place of the deleted language, insert "Landlord notifies
Tenant in writing of the estimated time period required to complete the repair
of the Premises or the Building".

                                       7
<PAGE>   47

        At the end of this Section, insert "In addition, Landlord shall have the
option to terminate this Lease if the Premises or the Building is destroyed or
damaged by fire or other casualty, regardless of whether the casualty is insured
against under this Lease, and Landlord reasonably determines that the total cost
of repairing or restoring the premises or the Building, as applicable, is at
least One Hundred Thousand Dollars ($100,000) more than the sum of the amount
insurance proceeds actually received by Landlord and the amount of any
applicable deductible."

21.     Page 5 - Section 11.e:

        Line 5, after the word "Term," insert "; provided, however, if Tenant
has an option to extend the term of this Lease, Landlord may not terminate this
Lease following a casualty pursuant to this Section 11.e if Tenant exercises
such option within ten (10) days after Landlord notifies Tenant in writing of
Landlord's election to terminate this Lease."

        Line 7, after the first occurrence of the word "repair", insert ", but
in no event more than ninety (90) days after the date of the casualty".

22.     Page 5 - Section 11.g:

        Add the following subsection:

        "g. Tenant's Right to Terminate. If the Premises are condemned or
        damaged by any peril and Landlord does not elect to terminate this Lease
        or is not entitled to terminate this Lease pursuant to the terms hereof,
        then Tenant shall have the option to terminate this Lease if the
        Premises are damaged or destroyed and Landlord reasonably determines
        that it would require more than two hundred seventy (270) days to repair
        or restore the Premises from after the date on which Tenant notifies
        Landlord in writing of the casualty. Landlord shall notify Tenant in
        writing within ninety (90) days after Tenant notifies Landlord of the
        casualty of Landlord's estimate as to the length of time required in
        order to repair or restore the Premises."

23.     Page 6 - Section 13.a:

        Line 1, before the word "Tenant", insert "Except to the extent caused by
Landlord's or Landlord's agents' or contractors' gross negligence or willful
misconduct,".

24.     Page 6 - Section 13.i:

        At the end of Section 13, add the following Section:

        "i. Tenant's Indemnity. Landlord shall indemnify, defend, protect and
        hold harmless Tenant from all losses, damages, liabilities, judgments,
        actions, claims, payments, costs and expenses, including reasonable
        attorneys' and consultants'

                                       8
<PAGE>   48

fees, arising from the negligence or willful misconduct of Landlord or its
agents, contractors, licensees or invitees, or Landlord's breach of any its
obligations under this Lease."

25.     Page 7 - Section 14:

        Line 1, before the word "If", insert:

        "a. CONSENT REQUIRED. Tenant shall not assign, mortgage, pledge or
        otherwise transfer this Lease, in whole or in part (each hereinafter
        referred to as an "assignment"), nor sublet or permit occupancy by any
        party other than Tenant of all or any part of the Premises (each
        hereinafter referred to as a "sublet" or "subletting"), without the
        prior written consent of Landlord in each instance, which consent shall
        not be unreasonably withheld. Landlord may consent to any subsequent
        assignment or subletting, or any amendment to or modification of this
        Lease with the assignees of Tenant, without notifying Tenant or any
        successor of Tenant, and without obtaining its or their consent thereto,
        and such action shall not relieve Tenant or any successor of Tenant of
        any liability under this Lease. Tenant shall reimburse Landlord for all
        reasonable legal fees and other expenses incurred by Landlord in
        connection with any request by Tenant for consent to an assignment or
        subletting.

        b.    NOTICE OF INTENT TO ASSIGN OR SUBLET."

        Line 2, in place of the deleted number, insert "thirty (30)".

        Lines 10 and 11, in place of the deleted percentage, insert [*].

        Line 11, after the word "sublet", insert "(after subtracting Tenant's
reasonable costs for attorneys' fees, brokerage commissions and improvements
constructed or paid for by Tenant in connection with the assignment or
subletting)".

        At the end of this Section, insert:

        "c. WITHHOLDING CONSENT. Without limiting other situations in which it
        may be reasonable for Landlord to withhold its consent to any proposed
        assignment or sublease, Landlord and Tenant agree that it shall be
        reasonable for Landlord to withhold its consent in any one (1) or more
        of the following situations: (1) in Landlord's reasonable judgment, the
        proposed subtenant or assignee or the proposed use of the Premises would
        detract from the status of the Building, generate vehicle or foot
        traffic, parking or occupancy density materially in excess of the amount
        customary for the Building or result in a materially greater use of the
        elevator, janitorial, security or other Building services (e.g., HVAC,
        trash disposal and sanitary sewer flows) than is customary for the
        Building; (2) in Landlord's reasonable judgment, the creditworthiness of
        the proposed subtenant or assignee does not meet the credit standards
        applied by Landlord in considering other tenants for the lease of space
        in the Building on comparable terms, or

-----------------------
[*]  Information redacted pursuant to a confidential treatment request
     throughout this exhibit.

                                       9
<PAGE>   49

        Tenant has failed to provide Landlord with reasonable proof of the
        creditworthiness of the proposed subtenant or assignee; (3) in
        Landlord's reasonable judgment, the business history, experience or
        reputation in the community of the proposed subtenant or assignee does
        not meet the standards applied by Landlord in considering other tenants
        for occupancy in the Building; (4) the proposed assignee or subtenant is
        a governmental entity, agency or department or the United States Post
        Office; or (5) the proposed subtenant or assignee is a then existing or
        prospective tenant of the Building.

        d. EXECUTED AGREEMENT. No sublease or assignment shall be valid, nor
        shall any subtenant or assignee take possession of the Premises, until a
        fully executed counterpart of the sublease or assignment has been
        delivered to Landlord and Landlord, Tenant and the applicable assignee
        or subtenant have entered into a consent to assignment or sublease in a
        form reasonably acceptable to Landlord.

        e. WAIVER. Notwithstanding any assignment or sublease, or any
        indulgences, waivers or extensions of time granted by Landlord to any
        assignee or sublessee, or failure by Landlord to take action against any
        assignee or sublessee, Tenant waives notice of any default of any
        assignee or sublessee and agrees that Landlord may, at its option,
        proceed against Tenant without having taken action against or joined
        such assignee or subleases, except that Tenant shall have the benefit of
        any indulgences, waivers and extensions of time granted to any such
        assignee or sublessee.

        f. NO CONSENT REQUIRED. Notwithstanding anything to the contrary
        contained in this Lease, Tenant may, without Landlord's prior written
        consent, and without being subject to any bonus rent provisions, sublet
        the Premises or assign the Lease to (i) a subsidiary or parent
        corporation controlling, controlled by or under common control with
        Tenant, (ii) a successor corporation by merger, consolidation,
        nonbankruptcy reorganization or government action, or (iii) a purchaser
        of all or substantially all of Tenant's assets. In addition, the sale of
        Tenant's capital stock through a private or public offering shall not be
        deemed to be an assignment for purposes of this Lease."

26.     Page 7 - Section 15.a:

        Line 4, in place of the deleted language, insert "within five (5) days
        after Tenant receives written notice of nonpayment".

        Lines 6, 7 and 8, in place of the deleted language, insert "thirty
        (30)".

        Line 10, in place of the deleted word, insert "made a general assignment
        of".

                                       10
<PAGE>   50

27.     Page 7 - Section 15.b(i):

        Line 3, in place of the deleted language, insert "In such event,
Landlord shall have the remedy described in California Civil Code Section 1951.4
(Landlord may continue this Lease in effect after Tenant's breach and
abandonment and recover Rent as it becomes due, if Tenant has the right to
sublet or assign, subject only to reasonable limitations), or any successor
statute."

28.     Page 7 - Section 15.b(ii):

        Paragraph 2, line 2, in place of the deleted percentage, insert "ten
percent (10%)".

29.     Page 8 - Section 16:

        Line 1, after the word "Lease", insert "beyond applicable notice and
cure periods".

        Line 3, after the word "Lease", insert "beyond applicable notice and
cure periods".

30.     Page 8 - Section 17:

        At the end of this Section, insert "Within seven (7) days after Landlord
receives Security Deposit from Tenant, as defined in Basic Lease Information,
Landlord shall deposit initial Security Deposit into an interest bearing money
market account. All interest shall be added to and become a part of Tenant's
Security Deposit. Any amount not applied in accordance with this Section 17
shall be refunded to Tenant, with all accrued but unpaid interest thereon,
within fifteen (15) days after the expiration or termination of this Lease."

31.     Page 9 - Section 18:

        Line 3, after the word "tear", insert "and casualty not caused by Tenant
or Tenant's employees, agents, representatives, contractors, assignees or
subtenants".

32.     Page 9 - Section 19:

        At the end of this Section, insert "In addition, Tenant shall indemnify,
protect, defend and hold harmless Landlord for all losses, expenses and damages,
including any consequential damages incurred by Landlord, as a result of Tenant
failing to surrender the Premises to Landlord and vacate the Premises by the end
of the Term."

33.     Page 9 - Section 20:

        Line 1, after the word "times", insert "upon twenty-four (24) hours'
prior notice (except in an emergency whereupon no prior notice shall be
required)".

                                       11
<PAGE>   51

34.     Page 9 - Section 21:

        At the end of this Section, insert "Tenant shall not install any sign in
or on the Premises, Building, or Property without the prior written consent of
Landlord. Landlord shall not unreasonably withhold its consent with respect to
interior signs that are not visible from the outside of the Building. With
respect to all other signs, Landlord may withhold or grant its consent in its
sole discretion. Landlord, at Tenant's cost and expense, shall provide Tenant
with signage on Tenant's suite and identify Tenant in the Building's directory,
all in accordance with Landlord's signage program for the Building. Subject to
(i) the approval of Landlord (which approval shall not be unreasonably withheld,
and the City, and (ii) Tenant obtaining all other necessary approvals, Tenant
shall have the exclusive right to the exterior signage on the Building;
provided, however, that Tenant's right to exterior signage shall terminate if at
any time during the Term Tenant to leases less than 37,500 rentable square feet
of space in the Building and occupies less than 24,000 rentable square feet of
space in the Building. If Tenant installs any signs in or on the Building or the
Property, Tenant shall remove all of Tenant's signs prior to the expiration of
the Term or earlier termination of this Lease and shall return the Premises,
Building, and Property to their condition existing immediately prior to the
placement or erection of Tenant's sign or signs."

35.     Page 9 - Section 23.a:

        Line 4, in place of the deleted language, insert "twenty (20)"

        Line 4, after the word "all", insert "commercially reasonable".

36.     Page 10 - Section 23.b:

        Line 2, in place of the deleted language, insert "twenty (20)".

37.     Page 10 - Section 23.c:

        At the end of this Section, insert "Notwithstanding anything to the
contrary in this Section 23, the subordination of Tenant's rights and interest
under the Lease to any mortgage or deed of trust hereafter affecting the
Property, the Building or the Premises, shall be contingent upon Tenant's having
received from any such mortgagee or beneficiary the mortgagee's or beneficiary's
standard form of recognition agreement which provides, among other things, that,
so long as Tenant is not in default under the terms of this Lease beyond any
applicable notice and cure periods, Tenant's rights under and interest in this
Lease shall not be disturbed in the event of any foreclosure of any such
mortgage or deed of trust and Tenant shall receive all of the rights and
services provided for under this Lease. Landlord shall, prior to the
Commencement Date, use commercially reasonable efforts to obtain from all ground
lessors or mortgagees holding a security interest in the Property, the Building
or the Property, a commercially reasonable form of recognition agreement which
provides, among other things, that, so long as Tenant is not in default under
the terms of this Lease beyond any applicable notice and cure periods, Tenant's
rights under and interest in this Lease shall not be disturbed in the event of
any foreclosure of

                                       12
<PAGE>   52

any such mortgage or other security instrument and Tenant shall receive all of
the rights and services provided for under this Lease."

38.     Page 10 - Section 27:

        Line 4, in place of the deleted language, insert "A party shall be
deemed to have prevailed in any action (without limiting the definition of
prevailing party) if such action is dismissed upon the payment by the other
party of the amounts allegedly due or the performance of obligations which were
allegedly not performed, or if such party obtains substantially the relief
sought by such party in the action, regardless or whether such action is
prosecuted to judgment."

39.     Page 10 - Section 28:

        Line 5, after the word "Lease", insert "as a result of Tenant's dealing
with such other broker or individual".

40.     Page 11 - Section 30:

        At the end of this Section, insert "If Tenant is unable to use the
Premises for the permitted use hereunder as a result of any interruption,
failure or stoppage of utilities to the Premises (except to the extent caused by
Tenant or its agents, representatives or invitees), and such interruption
continues for fifteen (15) consecutive calendar days, then Tenant shall be
entitled to an equitable abatement of rent to the extent of the interference
with Tenant' s use of the Premises occasioned thereby. If the interference
persists for more than ninety (90) consecutive calendar days, then Tenant shall
have the right to terminate the Lease by written notice to Landlord within ten
(10) days after the expiration of the ninety (90) day period referenced above."

41.     Page 11 - Section 32:

        Line 3, after the word "Landlord", insert "and Tenant".

42.     Sections 40 - 44:

        The following Sections are incorporated into this Lease:

        "40. Option to Lease Additional Space.

        a. Grant of Option. Provided that Tenant is not in default under the
        terms of this Lease beyond any applicable notice and cure period, during
        the period commencing on the date of this Lease and ending on the
        Commencement Date (the "Option Term"), Landlord grants to Tenant the
        option (the "Option") to lease

                                       13
<PAGE>   53

        additional space on the ground floor of the Building subject to the
        terms and conditions provided below.

        b. Commencement of Option. If at any time during the Option Term
        Landlord proposes to lease to a bona-fide third party (i) more than
        fifty percent (50%) of the total rentable square footage of space on the
        ground floor of the Building or (ii) a portion of the rentable square
        footage of space on the ground floor of the Building which, when added
        to the rentable square footage of space on the ground floor of the
        Building previously leased by Landlord to a third party other than
        Tenant, would equal fifty percent (50%) or more of the total rentable
        square footage of space on the ground floor of the Building, Landlord
        shall notify Tenant in writing ("Landlord's Option Notice") of such
        event. Landlord's Option Notice shall state both (i) the amount of space
        which Landlord proposes to lease to the third party (the "Offered
        Space") and (ii) the total amount of space which is then available for
        lease on the ground floor of the Building (the "Aggregate Available
        Space").

        c. Exercise of Right. Tenant shall have ten (10) days after Landlord
        delivers to Tenant Landlord's Option Notice in which to notify Landlord
        in writing of Tenant's election to lease either the Offered Space or the
        Aggregate Available Space on all of the terms and conditions set forth
        in this Lease, except (i) the term of Tenant's lease of the Offered
        Space or the Aggregate Available Space, as applicable, shall commence on
        the date Landlord delivers possession of the Offered Space or Aggregate
        Available Space to Tenant and shall expire on the date upon which the
        term of this Lease expires, (ii) Landlord shall not be obligated to
        perform any work in the Offered Space or the Aggregate Available Space,
        and (iii) Landlord shall not be obligated to construct any tenant
        improvements in the Offered Space or the Aggregate Available Space, and
        (iv) Landlord shall provide Tenant with a tenant improvement allowance
        in an amount equal to [*] per rentable square foot for general purpose
        office improvements constructed in the Offered Space or the Aggregate
        Available Space. If Tenant fails to notify Landlord in writing of its
        election to lease either the Offered Space or the Aggregate Available
        Space within the ten (10) day period referenced above, then (a) Tenant's
        Option shall terminate and be of no further force or effect and (b)
        Landlord shall have the right to lease the Offered Space or the
        Aggregate Available Space to any person or entity other than Tenant.

        d. Amendment. Within fourteen (14) days after Tenant exercises its
        Option, Landlord and Tenant shall amend this Lease to include the
        Offered Space or the Aggregate Available Space, as applicable, as part
        of the Premises.

        41.   Right of First Offer.

        a. Right of Tenant. Provided that Tenant is not in default under the
        terms of this Lease beyond any applicable notice and cure period, during
        the period commencing on September 1, 1999 and continuing until the
        expiration (and not

-----------------------
[*]  Information redacted pursuant to a confidential treatment request
     throughout this exhibit.

                                       14
<PAGE>   54

        including any extensions) or earlier termination of this Lease (the
        "Right of First Offer Term"), Tenant shall have a right of first offer
        to lease any Additional Space Increment. The Additional Space Increments
        subject to this right of first offer shall not include any space which
        is subject to expansion or extension rights of then existing tenants.

        b. Commencement of Right. If at any time during the Right of First Offer
        Term Landlord desires to offer to lease an Additional Space Increment to
        a third party tenant, Landlord shall notify Tenant in writing
        ("Landlord's Offer Notice") of Landlord's intent to lease the Additional
        Space Increment which notice shall specify all of the terms upon which
        Landlord proposes to lease the Additional Space Increment to Tenant
        (with the remainder of the terms to be those set forth in this Lease,
        but excluding any obligation for Landlord to perform any work in the
        Additional Space Increment, construct any tenant improvements or provide
        Tenant with any tenant improvement allowance).

        c. Exercise of Right. Tenant shall have ten (10) days after Tenant's
        receipt of Landlord's Offer Notice in which to notify Landlord in
        writing ("Tenant's Offer Notice") of its election to lease the
        Additional Space Increment on the terms set forth in Landlord's Offer
        Notice. If Tenant accepts Landlord's offer, the parties shall amend this
        Lease to add the Additional Space Increment as part of the Premises
        within fourteen (14) days after Tenant accepts Landlord's offer. If
        Tenant fails to deliver Tenant's Offer Notice to Landlord within the ten
        (10) day period referenced above, or Landlord and Tenant fail to enter
        into a lease for the Additional Space Increment within fourteen (14)
        days after Tenant delivers Tenant's Offer Notice to Landlord, then
        Tenant's rights under this Section 41 with respect to the Additional
        Space Increment shall terminate and be of no further force or effect.

        42. Right to Extend Term.

        a. Grant. Landlord hereby grants to Tenant one (1) option (the "Option")
        to extend the term of this Lease, for an additional period of five (5)
        years (the "Option Term"), commencing when the original Term expires,
        upon the terms and conditions set forth in this Section 43. Tenant shall
        exercise the Option (if at all) by giving Landlord written notice of its
        election no earlier than three hundred sixty-five (365) days prior to
        the expiration of the original Term and no later than one hundred eighty
        (180) days' prior to the expiration of the original Term. Tenant shall
        not have the right to exercise the Option if Tenant is in default under
        the terms of this Lease. In addition, at Landlord's election, Tenant's
        exercise of the Option shall be null and void if Tenant is in default
        under the terms of this Lease as of the commencement of the Option Term.

        b. Base Rent. If this Option is exercised, the Base Rent for the
        Premises shall be [*] of the then current fair market monthly rent
        ("Fair Market Rent") for the Premises as of the commencement date of the
        Option Term, as determined by the agreement of the parties or, if the
        parties cannot agree

-----------------------
[*]  Information redacted pursuant to a confidential treatment request
     throughout this exhibit.

                                       15
<PAGE>   55

        within one hundred twenty (120) days prior to the commencement of the
        Option Term, then by an appraisal. All other terms and conditions
        contained in the Lease and this Addendum, as the same may be amended
        from time to time by the parties in accordance with the provisions of
        the Lease, shall remain in full force and effect and shall apply during
        the Option Term (except that Tenant shall have no further options to
        extend the Term).

        c. Appraisal. If it becomes necessary to determine the Base Rent for the
        Premises during the Option Term by appraisal, then, on or before the one
        hundred tenth (110th) day prior to the expiration of the original Term,
        Landlord and Tenant shall each deliver to the other party a written
        notice setting forth its good faith estimate of the Base Rent for the
        Premises during the Option Term (collectively referred to as the
        "Estimates"). If the higher of such Estimates is not more than one
        hundred five percent (105%) of the lower of such Estimates, then the
        Base Rent for the Premises during the Option Term shall be the average
        of the two (2) Estimates. If the Base Rent for the Premises during the
        Option Term is not resolved by the exchange of Estimates, Landlord and
        Tenant, within ten (10) days after the exchange of Estimates, shall each
        select an appraiser to determine which of the two (2) Estimates most
        closely reflects a Base Rent that is [*] of the Fair Market Rent. Each
        appraiser so selected shall be certified as an MAI appraiser or as an
        ASA appraiser and shall have had at least five (5) years experience
        within the previous ten (10) years as a real estate appraiser working in
        the Santa Rosa area, with working knowledge of current rental rates and
        practices. For purposes of this Lease, an "MAI" appraiser means an
        individual who holds an MAI designation conferred by, and is an
        independent member of, the American Institute of Real Estate Appraisers
        (or its successor organization, or in the event the is no successor
        organization, the organization and designation most similar), and an
        "ASA" appraiser means an individual who holds the Senior Member
        designation conferred by, and is an independent member of, the American
        Society of Appraisers (or its successor organization, or in the event
        there is no successor organization, the organization and designation
        most similar). Upon selection, Landlord's and Tenant's appraisers shall
        work together in good faith to agree upon which of the two (2) Estimates
        most closely reflects a Base Rent that is [*] of the Fair Market Rent.
        The Estimate chosen by such appraisers shall be binding on both Landlord
        and Tenant. If either Landlord or Tenant fails to appoint an appraiser
        within the ten (10) day period referred to above, the appraiser
        appointed by the other party shall be the sole appraiser for the
        purposes hereof. If the two (2) appraisers cannot agree upon which of
        the two (2) Estimates most closely reflects a Base Rent that is [*] of
        the Fair Market Rent within the twenty (20) days after their
        appointment, then, within ten (10) days after the expiration of such
        twenty (20) day period, the two (2) appraisers shall select a third
        appraiser meeting the aforementioned criteria. Once the third appraiser
        has been selected as provided for above, then, as soon thereafter as
        practicable but in any case within fourteen (14) days thereafter, the
        appraiser shall make a determination of which of the two (2) Estimates
        most closely reflects a Base Rent that is [*] of the Fair Market Rent
        and such Estimate shall be binding on both Landlord and

-----------------------
[*]  Information redacted pursuant to a confidential treatment request
     throughout this exhibit.

                                       16
<PAGE>   56

        Tenant. The parties shall share equally in the costs of the third
        arbitrator and of any experts retained by the third arbitrator. Any fees
        of any appraiser, counsel or experts engaged directly by Landlord or
        Tenant, however, shall be borne by the party retaining such appraiser,
        counsel or expert. In the event that the Base Rent for the Option Term
        has not been determined by the commencement of the Option Term, Tenant
        shall pay Base Rent upon the same terms and conditions in effect
        immediately prior to the Option Term until such time as the Base Rent
        for the Option Term has been determined. Upon such determination, the
        Base Rent for the Premises during the Option Term shall be retroactively
        adjusted to commencement of the Option Term. Upon the determination of
        the Base Rent for the Premises for the Option Term, Landlord and Tenant
        shall enter into a letter agreement to memorialize the agreement. If
        such adjustment results in an underpayment of Base Rent by Tenant,
        Tenant shall pay Landlord the amount of such underpayment within thirty
        (30) days after the determination thereof.

        d. Fair Market Rent. For purposes hereof, "Fair Market Rent" shall mean
        the arms length fair market annual rental rate per rentable square foot
        under new leases (excluding renewals) entered into on or about the date
        on which the Fair Market Rent is being determined hereunder for space
        comparable to the Premises in the Building and office buildings
        comparable to the Building. The determination of Fair Market Rent shall
        take into account any material economic differences between the terms of
        this Lease and any comparison lease, such as rent abatements,
        construction costs, tenant improvement allowances, brokerage commissions
        (or the absence thereof), and different base years, and other
        concessions, and the manner, if any, in which the Landlord, under any
        such lease is reimbursed for operating expenses and taxes. The
        determination of Fair Market Rent shall take into consideration any
        reasonably anticipated changes in the Fair Market Rent from the time
        such Fair Market Rent is being determined and the time such Fair Market
        Rent will become effective under this Lease.

        43. Dish/Antenna.

        a. Right to Install. During the term of this Lease, Tenant shall have
        the right to install, maintain and operate a satellite dish or roof-top
        antenna (the "Dish/Antenna") on the roof of the Building subject to
        Landlord's and Landlord's architect's and/or engineer's approval of
        Tenant's plans and specifications for the Dish/Antenna, the manner in
        which the Dish/Antenna is to be attached to the roof of the Building,
        and the manner in which any cables are run to and from the Dish/Antenna,
        such approval not to be unreasonably withheld. Landlord shall designate
        the exact location on the roof where the Dish/Antenna is to be located.
        Landlord, at its sole cost and expense, reserves the right to relocate
        the Dish/Antenna to another location on the roof of the Building at any
        time during the term of this Lease. Tenant shall be solely responsible
        for obtaining all of the necessary governmental and regulatory approvals
        with respect to the installation and operation of the Dish/Antenna and
        for the cost of installing, operating, maintaining and removing the
        Dish/Antenna.

                                       17
<PAGE>   57

        b. Interference. Tenant agrees to only install equipment of types and
        frequencies which will not cause unreasonable interference to Landlord
        or any other tenants of the Building. If Tenant's equipment causes such
        interference, Tenant will change the frequency on which it transmits
        and/or receives and take any other steps necessary to eliminate the
        interference. If the interference cannot be eliminated within a
        reasonable period of time in Landlord's reasonable judgment, Tenant
        shall remove the Dish/Antenna from the Building.

        c. Maintenance. Tenant shall, at its sole cost and expense and at its
        sole risk, install, operate and maintain the Dish/Antenna in a good and
        workmanlike manner, and in compliance with all Building electric,
        communication and safety codes, laws, ordinances, standards, regulations
        and requirements, now in effect or hereafter promulgated.

        d. Removal. Tenant shall remove the Dish/Antenna at its own expense at
        the expiration or earlier termination of this Lease. Tenant shall repair
        any damage caused by Tenant's removal of the Dish/Antenna, including the
        patching of any holes to match, as closely as possible, the color
        surrounding the area where the equipment and appurtenances were
        attached.

        e. Exclusive Right. Provided that Tenant is not in default under the
        terms of this Lease beyond any applicable notice and cure period,
        Landlord shall not lease space on the roof of the Building to a provider
        of telecommunication, video, data or related services ("Communication
        Services") who is not a tenant in the Building for the purposes of
        providing Communication Services to unaffiliated tenants, occupants or
        licensees of another building.

        44. Landlord Estoppel. Within ten (10) days after Tenant's written
        request, Landlord shall execute the Landlord Waiver and Estoppel
        Certificate attached hereto as Exhibit G or such other documents
        reasonably acceptable to Landlord to evidence Landlord's waiver of any
        right, title, lien or interest in Tenant's personal property, equipment
        and trade fixtures (collectively, "Tenant's Personal Property") and give
        Tenant's lenders holding a security interest or lien on Tenant's
        Personal Property reasonable rights or access to the Premises to remove
        such personal property; provided, however, Landlord's waiver of its lien
        and grant of access to Tenant's lenders shall be subject to and
        conditioned upon the lender agreeing in writing (i) to indemnify, defend
        and hold harmless Landlord from any and all claims, losses, liability,
        costs or damages (including reasonable attorneys' fees) incurred by
        Landlord in connection with the lender's removal of Tenant's Personal
        Property and entry on the Premises or Landlord's property, (ii) to
        restore any portion of the Premises damaged by such lender's removal of
        Tenant's Personal Property to its condition existing immediately prior
        to the installation of the Tenant's Personal Property, (3) not to damage
        any portion of the Premises or any improvement of which the Premises are
        a part, and (4) not to conduct any sales or auctions on the Premises.

                                       18
<PAGE>   58

IN WITNESS WHEREOF, the parties have executed this Addendum to Lease on the
date(s) set forth below, as of the day and year first above written.

                                       LANDLORD:
                                       Stony Point East,
                                          a California general partnership

                                       By:  /s/  JAMES BRECHT
                                          ------------------------------------
                                          James Brecht
                                          Its Authorized Representative
Date  4/26/99
     ---------

                                       TENANT:
                                       Advanced TelCom Group,
                                           a Delaware corporation

                                       By:   /s/  C. G. RUDOLPH
                                          ------------------------------------

                                       Its:       Chairman & CEO
                                           -----------------------------------

Date  4/26/99
     ---------

                                       19

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