Document:

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                                                                   EXHIBIT 10.12

                            SUB-SUBLEASE AGREEMENT
                            ----------------------

          THIS SUB-SUBLEASE AGREEMENT ("Sublease") is made and entered into as
of this 5 day of June 1998, by and between STRATEGIC INTERACTIVE GROUP, INC.
("Sublessor") and ALLEGIANCE, TELECOM, INC., a Delaware corporation
("Sublessee").

                                    RECITALS
                                    --------

          This Sublease is made with regard to the following facts:

               A.  Tesseract Corporation ("Tesseract") is the Tenant under the
          Office Lease dated November 22, 1991 with Mosten Management Company,
          Inc., as Landlord ("Landlord"), pursuant to which Tesseract leased
          the 19th, 20th and 21st floors of 475 Sansome Street, San Francisco,
          California (hereinafter the "Master Lease"). A copy of the Master
          Lease is attached hereto as Exhibit A. The Master Lease has been
                                      ---------
          amended by that certain letter agreement dated August 13, 1997, by and
          among Landlord, Tesseract and Sublessor. A copy of such letter
          agreement is attached hereto as Exhibit B. The term "Master Lease"
                                          ---------
          shall include Exhibit B.
                        ---------

               B.  Sublessor is the sublessee under the Sublease Agreement dated
          August 21, 1997, in which Tesseract is the sublessor, and pursuant to
          which Sublessor subleased the entire 20th floor of 475 Sansome Street,
          San Francisco, California, from Tesseract Corporation. A copy of the
          Sublease Agreement is attached hereto as Exhibit C and is hereinafter
                                                   ---------
          referred to as the "SIG Sublease."

               C.  Sublessee desires to sub-sublease from Sublessor the portion
          of the 20th floor of 475 Sansome Street, San Francisco, California,
          which is marked by cross hatching on the diagram attached to this
          Sublease as Exhibit D (the "Subleased Space"), and Sublessor has
                      ---------
          agreed to sub-sublease the Subleased Space to Sublessee on the terms,
          covenants and conditions stated in this Sublease.

                                   AGREEMENT
                                   ---------

          NOW, THEREFORE, in consideration of the mutual covenants set forth
herein and for other good and valuable consideration, the receipt and
sufficiency of which are acknowledged by the parties, Sublessor and Sublessee
(collectively, the "Parties") agree as follows:

                                       1
<PAGE>

          1.   PROVISIONS CONSTITUTING SUBLEASE.
               --------------------------------

          This Sublease is subject to all of the terms and conditions of the
Master Lease, except Paragraphs 2, 3(a), 3(b), 9, 26, 42, 43, 44, 45, 46, 47,
48, 49, 50, and 52, and Exhibit C, and the SIG Sublease, except Paragraphs 4, 5,
6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20 and 21, and Exhibit "C",
and Sublessee shall perform the obligations of Tesseract and Sublessor under the
Master Lease and SIG Sublease, to the limited extent said terms and conditions
are applicable to the Subleased Space.

          Except as otherwise provided herein with respect to the Sublessee,
each party hereto agrees to perform and comply with the terms, provisions,
covenants and conditions of the Master Lease and the SIG Sublease, and not to do
or permit anything to be done which would result in a default under the Master
Lease or the SIG Sublease to the extent binding on such party, or cause the
Master Lease or the SIG Sublease to be terminated or forfeited.

          Any insurance carried by Landlord, Sublessor, or Sublessee with
respect to the Subleased Space and property therein or occurrences thereon
shall, if it can be so written without additional premium, or with an additional
premium which the requesting party agrees to pay, include a clause or
endorsement denying to the insurer right of subrogation against that party to
the extent rights have been waived by the insured prior to occurrence of injury
or loss. Each party, notwithstanding any provisions of this Sublease to the
contrary, hereby waives any rights of recovery against the other for property or
claims covered by insurance containing such a clause or endorsement to the
extent of the indemnification received thereunder.

          Notwithstanding any provision of the Master Lease or the Sublease to
the contrary, Sublessor shall be and remain liable for all of its obligations
under the SIG Sublease.

          Sublessee recognizes that Sublessor is not in a position to render any
of the services or to perform any of the obligations required of Landlord by the
terms of the Master Lease. Sublessee agrees that performance by Sublessor of its
obligations under this Sublease is conditioned on performance by the Landlord of
its corresponding obligations under the Master Lease, and Sublessor will not be
liable to Sublessee for any default of the Landlord under the Master Lease.
Notwithstanding the above provisions, Sublessor will use diligent commercially
reasonable efforts to enforce Tesseract's and Landlord's obligations under the
SIG Sublease and the Master Lease respectively.

                                       2
<PAGE>

          Sublessee will not have any claim against Sublessor based on the
Landlord's failure or refusal to comply with any of the provisions of the Master
Lease unless that failure or refusal is a result of any act or omission of
Sublessor or any employee, agent or contractor of Sublessor. Despite the
Landlord's failure or refusal to comply with any of those provisions of the
Master Lease, this Sublease will remain in full force and effect and Sublessee
will pay the base rent and all other charges provided for in this Sublease
without any abatement, deduction or setoff. However, Sublessee will receive its
prorata share of any rent abatement in the event of a condemnation or casualty
affecting the use of the Subleased Space, to the extent the Subleased Space is
rendered unusable by Sublessee in the conduct of its business. In the event
Sublessor receives the benefit of any other rent abatement under the SIG
Sublease, Sublessee will receive its share of such abatement to the extent that
the event giving rise to the abatement affects the Subleased Space.

          Sublessor represents and warrants that as of the date of this
Sublease, (a) the SIG Sublease is unmodified and in full force and effect, (b)
to the best of Sublessor's actual knowledge, the Sublessor has no offsets,
defenses or counterclaims against Tesseract under the SIG Sublease, (c) there is
no default or event of default by Sublessor or Tesseract under the SIG Sublease
or any event which with the giving of notice and/or the passage of time, or
both, would constitute a default or event of default by Sublessor or Tesseract
under the SIG Sublease, (d) to the best of Sublessor's actual knowledge without
inquiry, there is no default or event of default by Tesseract under the Master
Lease, or any event which with the giving of notice and/or the passage of time,
or both, would constitute a default or event of default by Tesseract under the
Master Lease, and (e) Sublessor has not encumbered its interest in the SIG
Sublease. Sublessor further represents and warrants to the best of Sublessor's
actual knowledge without inquiry that as of the date of this Sublease, (a) the
Master Lease is unmodified and in full force and effect and (b) there is no
default or event of default by Landlord or Tesseract under the Master Lease or
any event which with the giving of notice and/or the passage of time, or both,
would constitute a default or event of default by Landlord or Tesseract under
the Master Lease.

          2.   SUBLEASED SPACE.
               ---------------

          Sublessor hereby sublets to Sublessee and Sublessee hereby sublets
from Sublessor the Subleased Space, subject to the terms, covenants and
conditions contained in this Sublease, together with the non-exclusive right,
if any, to use the Common Areas of the Building. The Parties agree that the
Subleased Space is to consist of approximately 8,059 rentable square feet.

                                       3
<PAGE>

The actual rentable area of the Subleased Space shall be determined by
Sublessor's architect promptly after Sublessor has completed construction of the
demising walls which will separate the Subleased Space from Sublessor's
premises. The rentable area of the Subleased Space shall be calculated and
certified to Sublessor and Sublessee by Sublessor's architect pursuant to the
Standard Method for Measuring Floor Area in Office Buildings (ANSI/BOMA Z65.l,
1996). "Rentable Square Feet" and "Rentable Footage" shall have the same meaning
as the term "Rentable Area." Promptly after Sublessor has received its
architect's calculation of the rentable area of the Subleased Space, Sublessor
shall send Sublessee a copy of the calculation. If the architect's calculation
reflects that the rentable area of the Subleased Space consists of more or less
than 8,059 rentable square feet, then Sublessor shall promptly prepare an
amendment to this Sublease to reflect the Rentable Area of the Subleased Space,
to make the necessary adjustments to the rent and Sublessee's percentage share
of Operating Costs and Property Taxes, and Sublessor and Sublessee shall execute
the amendment.

          3.   TERM.
               ----

          The term of this Sublease shall commence on July 1, 1998 (the
"Commencement Date") and end on April 30, 2002 (the "Expiration Date"), unless
sooner terminated as provided herein. If Sublessor for any reason cannot
deliver possession of the Subleased Space to Sublessee on the Commencement Date,
this Sublease shall not be void or voidable nor shall Sublessor be liable to
Sublessee for any damage resulting therefrom. Notwithstanding the foregoing, if
possession of the Subleased Space has not been delivered to Sublessee by August
1, 1998, Sublessee, at its option, at any time thereafter but prior to the
delivery of possession may terminate this Sublease by notice to Sublessor, and
Sublessor and Sublessee shall thereupon be released from all obligations under
this Sublease. No delay in delivery of possession shall operate to extend the
term hereof.

          4.   RENT.
               ----

          Sublessee shall pay to Sublessor, without deduction, set off, prior
notice or demand, as rent during the term of the Sublease, the sum of $23,84l.21
per month, payable monthly in advance in lawful money of the United States of
America on the first day of each month for the period starting with the
Commencement Date through the Expiration Date, unless sooner terminated. In the
event that pursuant to the procedure set forth in paragraph 2 above the rentable
area of the Subleased Space is determined to be different than 8,059 rentable
square feet, then (a) the rent hereunder shall be appropriately adjusted using
an annual rental rate of $35.50 per rentable square foot, and (b) Sublessee's
prorata share of increases in Building

                                       4
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Operating Expenses and Taxes shall be appropriately adjusted. The revised rent
and revised prorata share percentage figure shall be incorporated in the
amendment to this Sublease which is to be prepared by Sublessor in accordance
with paragraph 2 above. In the event that any month during the term of this
Sublease shall be less than a full month, the rent payable hereunder shall be
prorated for such month based upon the number of days this Sublease shall be in
effect during such month.

          Payment of rent and any other obligations under the Sublease shall be
delivered to Sublessor's address for notices under the Sublease on or before the
first day of each calendar month during the term. Sublessee agrees that, in the
event Sublessee fails to pay any sum of money due hereunder within the
applicable grace period, if any, Sublessor shall be entitled to collect late
charges and interest as set forth in paragraphs 3(d) and 3(e) of the Master
Lease.

          5.   TENANT IMPROVEMENTS.
               -------------------

          Prior to delivery of the Subleased Space to Sublessee, Sublessor, at
its expense, shall install demising walls between Sublessor's premises and the
Subleased Space in a good and workmanlike manner in accordance with all
applicable laws, so that the two areas are physically separated. On Sublessee's
side of the demising walls the demising walls shall be painted by Sublessor so
as to reasonably match the existing painted surfaces adjacent to the walls.
With the exception of the work described in the two immediately preceding
sentences, Sublessee acknowledges that the Subleased Space shall be delivered to
Sublessee in its existing "As Is" condition. Sublessee agrees that any and all
additional and subsequent improvement work in the Subleased Space must first be
approved in writing by Landlord and Tesseract and consented to by Sublessor.

          6.   BASE YEAR AND OPERATING EXPENSE ADJUSTMENT.
               ------------------------------------------

          For purposes of this Sublease, the "Base Year" shall be calendar year
1998. Sublessee agrees to pay Sublessor, at the same time and in the same manner
which Tesseract is required to pay Landlord Escalation Rent under paragraph 4 of
the Master Lease, Sublessee's prorata share of increases in Building Operating
Expenses and property Taxes over the Base Year. Sublessee's prorata share is
2.59% of the Building square footage. Sublessor agrees to submit for Sublessee's
review all documentation provided by Tesseract and Landlord regarding Operating
Expenses and adjustments. Upon the written request of Sublessee to Sublessor,
Sublessor agrees to exercise any rights it may have to audit Landlord's
Operating Expenses and Taxes and to share the results with Sublessee. If only
Sublessee desires that Sublessor exercise such audit rights as it may have,

                                       5
<PAGE>

Sublessee shall pay the entire cost of any such audit upon written demand from
Sublessor. If both Sublessor and Sublessee desire to have Sublessor exercise
such audit rights as it may have, then the cost of such audit shall be paid
fifty percent (50%) by Sublessor and fifty percent (50%) by Sublessee upon
written demand from Sublessor. Any utility expenses passed on by Landlord for
operating equipment such as HVAC outside of business hours or normal usage as
these costs pertain to the Subleased Space shall be passed on to Sublessee to
the extent such additional services were used or requested by Sublessee.
Sublessor shall be entitled to reasonably estimate Sublessee's share of any such
costs.

          7.  DEPOSIT.
              -------

          Sublessee has deposited with Sublessor the sum of $47,682.42 (the
"Deposit"). The Deposit shall be held by Sublessor as security for the faithful
performance by Sublessee of all of the provisions of this Sublease to be
performed or observed by Sublessee. If Sublessee fails to pay rent or other
charges due hereunder after giving effect to any applicable cure period, or
otherwise defaults with respect to any provision of this Sublease, Sublessor may
use, apply or retain all or any portion of the Deposit for the payment of any
rent or other charge in default or the payment of any other sum to which
Sublessor has become obligated by reason of Sublessee's default, or to
compensate Sublessor for any actual loss or damage which Sublessor may suffer
thereby. If Sublessor so uses or applies all or any portion of the Deposit, then
within ten (10) business days after demand thereof or Sublessee shall deposit
cash with Sublessor in an amount sufficient to restore the Deposit to the full
amount thereof, and Sublessee's failure to do so shall be a material breach of
this Sublease. Sublessor shall not be required to keep the Deposit separate from
its general accounts. The Deposit, or so much thereof as has not theretofore
been applied by Sublessor, shall be returned, without payment of interest or
other increment for its use, to Sublessee (or, at, Sublessor's option, to the
last assignee, if any, of Sublessee s interest hereunder) at the expiration of
the term hereof, and after Sublessee has vacated the Subleased Space. No trust
relationship is created herein between Sublessor and Sublessee with respect to
the Deposit.

          8.  OPTION TO RENEW.
              ---------------

          Sublessee shall have no option to renew or extend the term of this
Sublease.

                                       6
<PAGE>

          9.  USE.
              ---

          Lessee shall use the Subleased Space for general office purposes and
for no other purpose without the prior written consent of Sublessor.

          Sublessee's business shall be established and conducted throughout the
term hereof in a first class manner. Sublessee shall not use the Subleased Space
for, or carry on, or permit to be carried on, any offensive, noisy or dangerous
trade, business, manufacture or occupation, nor permit any auction sale to be
held or conducted on or about the Subleased Space. Sublessee shall not do or
suffer anything to be done upon the Subleased Space which will cause structural
injury to the Subleased Space, or the building of which the same form a part.
The Subleased Space shall not be overloaded and no machinery, apparatus or other
appliance shall be used or operated in or upon the Subleased Space which will
injure, vibrate or shake the Subleased Space or the building of which it is
part. No use shall be made of the Subleased Space which will in any way impair
the efficient operation of the sprinkler system (if any) within the building
containing the Subleased Space. No musical instrument of any sort, or any noise
making device will be operated or allowed upon the Subleased Space for any
purpose. No use will be made of the Subleased Space which will increase the
existing rate of insurance upon the building in which the Subleased Space is
located, or cause a cancellation of any insurance policy covering the building
or any part thereof.

          If any act on the part of Sublessee or use of the Subleased Space by
Sublessee shall cause, directly or indirectly, any increase of Sublessor's
insurance expense, said additional expense shall be paid by Sublessee to
Sublessor upon demand. No such payment by Sublessee shall limit Sublessor in the
exercise of any other rights or remedies, or constitute a waiver of Sublessor's
right to require Sublessee to discontinue such act or use.

          10.  ENTRY BY SUBLESSOR.
               ------------------

          Sublessor reserves the reasonable right to inspect Sublessee's
Subleased Space during normal business hours and with prior notice to Sublessee.

          11.  REPAIR AND MAINTENANCE.
               ----------------------

          Sublessee shall use the Subleased Space and any other items which are
made available by Sublessor to Sublessee during the term of this Sublease in a
normal and business-like manner and keep the same in good order, condition and
repair, excluding normal wear and tear and casualty damage not attributable to

                                       7
<PAGE>

Sublessee. Sublessee shall return the Subleased Space and such items at the end
of the term to Sublessor in the same condition as on the Commencement Date,
excluding normal wear and tear and casualty damage not attributable to
Sublessee.

          12. ALTERATIONS.
              -----------

          Any and all alteration to the Subleased Space must be reviewed and
approved by Sublessor, Landlord and Tesseract prior to any such alteration being
performed. Landlord's approval is subject to the standards set forth in the
Master Lease. Sublessor agrees that it shall not unreasonably withhold or delay
its consent to any requested alterations, provided that Landlord consents to the
requested alterations. Sublessee is responsible for any Landlord-assessed costs
for alterations incurred at Sublessee's sole option, including any costs of the
removal of such alterations at the end of this Sublease as mandated by
Landlord.

          13. CONDEMNATION PROCEEDS.
              ---------------------

          Despite anything contained in the Master Lease to the contrary, as
between Sublessor and Sublessee only, in the event of condemnation of the
Subleased Space all condemnation awards received by Sublessor under the Master
Lease will be deemed to be the property of Sublessor, and Sublessor will have no
obligation to restore the Subleased Space. The foregoing shall not, however,
preclude the Sublessee from bringing a separate proceeding to recover the value
of any improvements or alterations made by the Sublessee, moving expenses and
the value of the unexpired term of this Sublease, provided that such recovery
does not diminish any award to Sublessor.

          14. TIME LIMITS.
              -----------

          The time limits provided for in the provisions of the Master Lease for
the giving of notice, making of demands, payment of any amount, performance of
any act, condition or covenant, or the exercise of any right, remedy or option,
are amended for the purposes of this Sublease, by shortening the same in each
instance by five (5) days, so that notices may be given, demands made, or
payments made, or any act, condition or covenant performed, or any right, remedy
or option hereunder exercised, by Sublessor or Sublessee, as the case may be,
within the time limit relating thereto contained in the Master Lease. If the
Master Lease allows only five (5) days or less for Sublessor to perform any act,
or to undertake to perform such act, or to correct any failure relating to the
Subleased Space or this Sublease, then Sublessee shall nevertheless be allowed
three (3) days to perform such act, undertake such act and/or correct such
failure.

                                       8
<PAGE>

          15. BROKERS.
              -------

          Sublessor and Sublessee warrant that Cushman & Wakefield acted as
Sublessor's only broker and that Partners National Real Estate Group, Inc. acted
as Sublessee's only broker. Sublessor shall pay Partners National Real Estate
Group, Inc. a real estate brokerage commission equal to $1.00 per rentable
square foot per year of the term of the Sublease, which shall be paid one-half
(1/2) upon mutual execution of this Sublease and one-half (1/2) upon occupancy
of the Subleased Space by Sublessee. Sublessee and Sublessor agree to indemnify
and hold each other harmless from and against any claims by any other broker,
agent or other person claiming a commission or other form of compensation by
virtue of having dealt with the indemnifying party with regard to this
subleasing transaction. The provisions of this paragraph shall survive the
termination of this Sublease.

          16. QUIET ENJOYMENT. So long as Sublessee is not in default beyond any
              ---------------
applicable cure period under this Sublease, Sublessee's peaceful and quiet
enjoyment of the Subleased Space shall not be disturbed on account of any act or
omission of Sublessor. The covenant of quiet enjoyment in the preceding sentence
specifically excludes any disturbance of Sublessee's possession of the Subleased
Space for any reason other than those set forth in the preceding sentence.

          17. NOTICES OF DEFAULT.
              ------------------

          Sublessor agrees to notify Sublessee of any event of default by
Sublessee hereunder, by Sublessor under the SIG Sublease, or Tesseract under the
Master Lease, within five (5) business days of Sublessor's receipt of a notice
of default.

          18. PARKING.
              -------

          Subject to all applicable restrictions placed upon Sublessor by
Landlord in the Master Lease, Sublessee shall be entitled to park three (3)
automobiles in the Building's parking garage in valet parking spaces from time
to time designated by Sublessor, subject to all of the rules and regulations
applicable to such parking as are promulgated by Landlord and the Garage
Operator (the "Operator") and to any restrictions or regulations at any time
imposed by the City and County of San Francisco on Landlord's ability to offer
such parking. The rental rate for such parking shall be equal to the rate from
time to time charged by the Operator and Landlord to the public for comparable
parking privileges in the Building and shall be paid directly by Sublessee to
the Operator as directed in the Operator's sole discretion.

                                       9
<PAGE>

          19.  RIGHT TO ASSIGN OR SUBLEASE.
               ---------------------------

          Subject to obtaining Landlord's and Tesseract's prior written consent,
to the extent required by the terms of the Master Lease and the SIG Sublease, as
the case may be, Sublessee shall have the right, with notice to Sublessor but
without Sublessor's consent, to assign this Sublease or to sublease the
Subleased Space to any entity controlled or under common control with Sublessee,
or to a corporation with which Lessee has merged or consolidated, or to any
corporation or other entity which acquires substantially all of Sublessee's
assets as a going concern. Except as provided in the preceding sentence,
Sublessee shall have no right to assign this Sublease or to further sublet all
or any portion of the Subleased Space.

          20.  ADDRESSES FOR NOTICES.
               ---------------------

          All notices of any kind required under the provisions of this Sublease
shall be mailed, postage prepaid by certified or registered mail, return receipt
requested, or by a nationally recognized overnight delivery service, addressed
as follows:

If to Sublessor:  Strategic Interactive Group, Inc.
                  Attn:  Kathleen O'Meara, Vice President
                  Director of Finance and Operations
                  The Prudential Tower
                  800 Boylston Street
                  Boston, MA 02199

If to Sublessee:  Allegiance Telecom, Inc.
                  Attn:  Patricia E. Koide
                  4 Westbrook Corporate Center, Suite 400
                  Westchester, IL 60154

                  with a copy to:

                  Swidler & Berlin, Chartered
                  Attn:  Jeffrey Scharff
                  3000 K Street, N.W.
                  Washington, D.C. 20007

          21. GENERAL PROVISIONS.
              ------------------

          21.1.  Severability. If any provision of this Sublease or the
                 ------------
application of any provision of this Sublease to any person or circumstance is,
to any extent, held to be invalid or unenforceable, the remainder of this
Sublease or the application of that provision to persons or circumstances other
than those as to which it is held invalid or unenforceable, will not be
affected, and each provision of this Sublease will be valid and be enforced to
the fullest extent permitted by law.

                                       10
<PAGE>

          21.2. Entire Agreement; Waiver. This Sublease constitutes the final,
                ------------------------
complete and exclusive statement between the parties to this Sublease pertaining
to the Subleased Space, supersedes all prior and contemporaneous understandings
or agreements of the parties, and is binding on and inures to the benefit of
their respective heirs, representatives, successors, and assigns. No party has
been induced to enter into this Sublease by, nor is any party relying on, any
representation or warranty outside those expressly set forth in this Sublease.
Any agreement made after the date of this Sublease is ineffective to modify,
waive, release, terminate, or effect an abandonment of this Sublease, in whole
or in part, unless that agreement is in writing, is signed by the parties to
this Sublease.

          21.3. Captions. Captions to the sections in this Sublease are included
                --------
for convenience only and do not modify any of the terms of this Sublease.

          21.4. Further Assurances. Each party to this Sublease will at its own
                ------------------
cost and expense execute and deliver such further documents and instruments and
will take such other actions as may be reasonably required or appropriate to
evidence or carry out the intent and purposes of this Sublease.

          21.5. Governing Law. This Sublease will be governed by and in all
                -------------
respects construed in accordance with the laws of the State of California.

          21.6. Consent of Landlord and Tesseract Corporation. The Landlord's
                ---------------------------------------------
and Tesseract Corporation's written consent to this Sublease is a condition
subsequent to the validity of this Sublease.

          21.7. Capitalized Terms. All terms spelled with initial capital
                -----------------
letters in this Sublease that are not expressly defined in this Sublease will
have the respective meanings given such terms in the Master Lease.

          21.8. Word Usage. Unless the context clearly requires otherwise, (a)
                ----------
the plural and singular numbers will each be deemed to include the other; (b)
the masculine, feminine, and neuter genders will each be deemed to include the
others; (c) "shall," "will," "must," "agrees," and "covenants" are each

                                       11
<PAGE>

mandatory; (d) "may" is permissive; (e) "or" is not exclusive; and (f)
"includes" and "including" are not limiting.

          21.9.  Amendment to SIG Sublease and Master Lease.
                 ------------------------------------------

              (a) Sublessor covenants and agrees that it will not amend or
modify the SIG Sublease, to the extent that such amendment or modification
affects this Sublease or the Subleased Space, without notice to and the prior
written consent of Sublessee.

              (b) Sublessor will use reasonable efforts to prevent any amendment
or modification to the Master Lease, to the extent such amendment or
modification affects this Sublease or the Subleased Space. Sublessor agrees to
give notice to Sublessee of any proposed amendment or modification to the Master
Lease which Sublessor receives.

          IN WITNESS WHEREOF, Sublessor and Sublessee have executed this Sub-
Sublease Agreement as of the date first set forth above.

SUBLESSOR:

STRATEGIC INTERACTIVE GROUP,
INC.

By: Kathleen O. O'Meara
   -----------------------------------------------
   Its VP, Director of Finance & Operations 6/5/99
      --------------------------------------------

SUBLESSEE:

ALLEGIANCE TELECOM, INC.,
a Delaware corporation

By: /s/ Patricia E. Koide
   -----------------------------------------------
   Its Sr VP
      --------------------------------------------

By: /s/ [ILLEGIBLE]^^
   -----------------------------------------------
   Its Secretary
      --------------------------------------------

                                       12
<PAGE>

Consent of Landlord:

          The undersigned, Landlord under the Master Lease, hereby consents to
the subletting of the Subleased Space on the terms and conditions contained in
this Sublease. This consent shall apply only to this Sublease and shall not be
deemed to be a consent to any other sublease.

          Landlord represents and warrants (a) the Master Lease is unmodified
and in full force and effect, and (b) there is no default or event of default by
Landlord or Tesseract under the Master Lease or any event which with the giving
of notice and/or the passage of time, or both, would constitute a default or
event of default by Landlord or Tesseract under the Master Lease.

MOSTEN MANAGEMENT COMPANY, INC.
a Delaware corporation

By: /s/ Stephen J. Spey
   -----------------------------
        STEPHEN J. SPEY

   Its VICE PRESIDENT
      --------------------------

Dated: 6/22        1998
      ------------,

Consent of Tesseract:

          Tesseract, as sublessor to Strategic Interactive Group, Inc., hereby
consents to the subletting of the Subleased Space on the terms and conditions
contained in this Sublease. This consent shall apply only to this Sublease and
shall not be deemed to be a consent to any other sublease.

          Tesseract covenants and agrees for the benefit of Strategic
Interactive Group, Inc. and Allegiance Telecom, Inc. that it will not amend or
modify the Master Lease or the SIG Sublease, to the extent that any such
amendment or modification affects this Sublease or the Subleased Space, without
notice to and the prior written consent of Allegiance Telecom, Inc.

          Tesseract represents and warrants (a) the Master Lease and the SIG
Sublease are unmodified and in full force and effect, (b) Tesseract has no
offsets, defenses or counterclaims against Landlord under the Master Lease, (c)
there is no default or event of default by Landlord or Tesseract under the
Master Lease or any event which with the giving of notice and/or the passage of
time, or both, would constitute a, default or event of default by Landlord or
Tesseract under the Master Lease, (d) there is no default or event of default by
Strategic Interactive Group, Inc. or Tesseract under the SIG Sublease or any
event which with the giving of notice and/or the passage of time, or both, would

                                       13
<PAGE>

constitute a default or event of default by Strategic Interactive Group, Inc. or
Tesseract under the SIG Sublease, and (e) Tesseract has not encumbered its
interest in the Master Lease.

TESSERACT CORPORATION,
a California corporation

By:/s/ Lee G. Larson
   ---------------------------
   Its Mgr., Real Estate
      ------------------------
Dated: 6/16        1998
      ------------,

                                       14
<PAGE>

                     TERMINATION OF SUB-SUBLEASE AGREEMENT
                     -------------------------------------

          THIS TERMINATION OF SUB-SUBLEASE AGREEMENT (this "Agreement") is made
and entered into as of December 7, 1999, by and between BRONNERCOM, LLC, a
Delaware limited liability company ("Sublessor"), and ALLEGIANCE TELECOM, INC.,
a Delaware corporation ("Sublessee").

                                   RECITALS
                                   --------

          A.   Sublessor's predecessor-in-interest, Strategic Interactive Group,
Inc., as sublessor, and Sublessee have entered into that certain Sub-Sublease
Agreement dated June 5, 1998 (the "Sublease"), with respect to certain office
premises on a portion of the 20th floor of 475 Sansome Street, San Francisco,
California, which are more particularly described in the Sublease (the
"Subleased Space"). Terms used herein that are defined in the Sublease shall
have the meanings therein defined.

          B.   The date of expiration of the term of the Sublease is April 30,
2002.

          C.   Sublessor and Sublessee desire to terminate the Sublease on the
conditions specified in this Agreement.

          NOW, THEREFORE, in consideration of the recitals and covenants herein
contained and other good and valuable consideration, the receipt and
sufficiency of which are hereby acknowledged, Sublessor and Sublessee hereby
agree as follows:

                                       1
<PAGE>

                                   AGREEMENT
                                   ---------

          1.   Termination of Sublease. Notwithstanding anything contained in
               -----------------------
the Sublease to the contrary, effective as of 11:59 p.m. on May 15, 2000 (the
"Effective Date"), (a) the term of the Sublease shall terminate and expire and
the Sublease shall be terminated; (b) Sublessee shall vacate and surrender
possession of the Subleased Space to Sublessor in broom-clean condition and
otherwise in the condition required by the Sublease; (c) Sublessee shall deliver
the keys to the Subleased Space to Sublessor and Sublessor shall accept the
surrender of the Subleased Space in such condition; and (d) Sublessee shall
remove any vehicles parked in the Building pursuant to the provisions of
paragraph 18 of the Sublease. Conditioned upon the surrender of the Subleased
Space on the Effective Date in the condition required hereby, Sublessor and
Sublessee shall be released from all obligations under the Sublease which have
not accrued as of the Effective Date, provided that all agreements in the
Sublease to indemnify or hold harmless shall survive with respect to matters
occurring prior to the Effective Date.

          2.   Sublessee's Right to Advance Effective Date. Sublessee shall
               -------------------------------------------
have the right to advance the Effective Date by giving Sublessor not less than
thirty (30) days prior written notice specifying a date prior to May 15, 2000
which Sublessee desires to become the Effective Date. In the event Sublessee
gives Sublessor notice advancing the Effective Date, the date

                                       2
<PAGE>

specified in such notice shall thereafter become the Effective Date for all
purposes under this Agreement, and the former Effective Date of May 15, 2000
shall thereafter be of no further force or effect.

          3.   Liquidated Damages. Sublessor and Sublessee acknowledge and agree
               ------------------
that Sublessor plans to occupy the Subleased Space immediately after Sublessee
vacates the Subleased Space and therefore it is extremely important to Sublessor
that Sublessee vacate the Subleased Space by no later than the Effective Date.
IN THE EVENT SUBLESSEE FAILS TO VACATE THE SUBLEASED SPACE IN THE CONDITION
REQUIRED BY THIS AGREEMENT BY NO LATER THAN THE EFFECTIVE DATE, THEN, IN LIEU OF
RENT UNDER THE SUBLEASE, SUBLESSEE SHALL PAY SUBLESSOR THE SUM OF TWO THOUSAND
DOLLARS ($2,000.00) PER DAY AS STIPULATED, FIXED, AGREED AND LIQUIDATED DAMAGES
(AND NOT AS A PENALTY) FOR EACH DAY AFTER THE EFFECTIVE DATE UNTIL SUBLESSEE HAS
FULLY COMPLIED WITH ITS OBLIGATIONS IN THE FIRST SENTENCE OF THIS PARAGRAPH 3.
THE PARTIES ACKNOWLEDGE THAT SUBLESSOR'S ACTUAL DAMAGE DUE TO SUCH FAILURE WOULD
BE EXTREMELY DIFFICULT OR IMPRACTICABLE TO DETERMINE. THEREFORE, BY PLACING
THEIR INITIALS BELOW, THE PARTIES ACKNOWLEDGE THAT THE AMOUNT OF THE LIQUIDATED
DAMAGES SPECIFIED HEREIN CONSTITUTES A REASONABLE ESTIMATE OF THE DAMAGE WHICH
WOULD BE SUFFERED BY SUBLESSOR AS A RESULT OF
                                       3
<PAGE>

SUBLESSEE'S FAILURE TO VACATE THE SUBLEASED SPACE IN THE CONDITION REQUIRED BY
THIS AGREEMENT BY NO LATER THAN THE EFFECTIVE DATE.

     Sublessor's Initials: M.K.B.  Sublessee's Initials: P.K.
                           ------                        ----

          4. Force Majeure.  If Sublessee's ability to vacate the Subleased
             -------------
Space and move into the new premises which Sublessee has leased at 505 Sansome
Street, 20th floor, San Francisco, California (the "New Premises") is made
impossible by any prevention, delay, or stoppage due to strikes, lockouts, labor
disputes, acts of God, inability to obtain services, labor or materials or
reasonable substitutes for those items, governmental actions, civil commotions,
fire or others casualty, and other causes beyond the reasonable control of
Sublessee (collectively, "Force Majeure") then, notwithstanding anything to the
contrary in this Agreement, the Effective Date shall be extended one day for
each day Sublessee cannot vacate the Subleased Space and move into the New
Premises due to Force Majeure, provided, however, in no event shall the
Effective Date be extended by Force Majeure for more than one hundred eighty
(180) days. Force Majeure shall not include failure by the New Premises landlord
or the landlord's contractor to complete improvements in the New Premises,
unless such failure is due to Force Majeure. Within five (5) days of Sublessee's
learning of Force Majeure which will extend the Effective Date, Sublessee shall
give Sublessor notice of the cause of Force Majeure and the

                                       4

<PAGE>

anticipated time of such extension, and shall thereafter also give Sublessor
notice promptly after Sublessee has determined the actual date on which
Sublessee will vacate the Subleased Space as a result of the extension of the
Effective Date by Force Majeure.

          5.   Payment of Rent and Other Sums.  Sublessee shall pay to Sublessor
               ------------------------------
when due all rent and other sums due under the Sublease through and including
the Effective Date.

          6.   Deposit.  Sublessor and Sublessee hereby acknowledge that
               -------
pursuant to the provisions of paragraph 7 of the Sublease, Sublessor holds the
sum of $47,682.42 (the "Deposit") as security for the faithful performance by
Sublessee of all provisions of the Sublease to be performed or observed by
Sublessee. Provided that through the Effective Date Sublessee has fully
performed and observed all of the provisions of this Agreement which Sublessee
is required to perform or observe, Sublessor shall promptly return the Deposit
to Sublessee in accordance with the provisions of paragraph 7 of the Sublessee,
less any amounts which Sublessor is entitled to retain under the Sublease or
paragraph 3 of this Agreement.

          7.   Sublessor's Right to Recover Possession.  In the event that
               ---------------------------------------
Sublessee has not vacated the Subleased Space by the Effective Date, as such
date may be advanced or extended as provided in this Agreement, Sublessor shall
have the immediate right to commence appropriate legal proceedings to recover
possession of the Subleased Space and to recover legal fees and

                                       5
<PAGE>

costs incurred by Sublessor in doing so.

          8.   Complete Agreement.  This Agreement represents the final
               ------------------
expression, and the complete and exclusive statement, of all of the oral and
written agreements, conditions, promises and covenants between Sublessor and
Sublessee with respect to the subject matter hereof, and this Agreement
supersedes all prior or contemporaneous oral and written agreements and
understandings between Sublessor and Sublessee with respect to the subject
matter covered hereby, including the Sublease.  This Agreement may only be
amended by an agreement in writing signed by both Sublessor and Sublessee.

          IN WITNESS WHEREOF, Sublessor and Sublessee have executed this
Termination of Sub-Sublease Agreement as of the date first above written.

SUBLESSOR:                              SUBLESSEE:

BRONNERCOM, LLC,                        ALLEGIANCE TELECOM, INC.,
a Delaware limited                      a Delaware corporation
liability company

By:   /s/ M.K. Beckingham               By:  /s/ Patricia E. Koide
   ----------------------                  ---------------------------

Name: /s/ M.K. Beckingham               Name: Patricia E. Koide
     --------------------                     ------------------------

Title: EVP & CFO                        Title: Senior Vice President
      -------------------                     ------------------------

Date:   12/7                            Date:      12/7, 1999
     -------------, 1999                      ------------------------

                                        By: /s/ Mark B. Tresnowski
                                           ---------------------------

                                        Name:  Mark B. Tresnowki
                                              ------------------------

                                        Title: Senior Vice President
                                              ------------------------
                                               & Corporate General
                                              ------------------------
                                               Counsel
                                              ------------------------
                                        Date:      12/7, 1999
                                              ------------------------

                                       6<PAGE>

                                                                   EXHIBIT 10.13

                              SUBLEASE AGREEMENT
                              ------------------

1.   PARTIES

     This SUBLEASE AGREEMENT ("Sublease") is entered into on this 21st of August
1997 by and between Tesseract Corporation ("Sublessor"), Strategic Interactive
Group, Inc. ("Sublessee"), and Bronner Slosberg Humphrey Inc. ("Parent"), as the
guarantor for Sublessee, as a sublease under the lease dated November 22, 1991,
entered into by Mosten Management Company, Inc., as Landlord, and Tesseract
Corporation as Tenant, attached as Exhibit A (the "Master Lease").

     In consideration of mutual covenants set forth herein and for other good
and valuable consideration, Sublessor and Sublessee (collectively, the
"Parties") agree as follows:

2.   PROVISIONS CONSTITUTING SUBLEASE

     This sublease is subject to all of the terms and conditions of the Master
Lease in Exhibit A and Sublessee shall perform the obligations of Sublessor and
Tenant in said lease, to the extent said terms and conditions are applicable to
the premises subleased pursuant to this sublease.

     Each party hereto agrees to perform and comply with the terms, provisions,
covenants and conditions of the Master Lease, and not to do or permit anything
to be done which would result in a default under the Master Lease, or cause the
Lease to be terminated or forfeited.

     Each party hereto agrees not to breach or violate any of the terms,
covenants and conditions contained in the Master Lease.

     Sublessor represents and covenants that Sublessor is not currently in
default of the Master Lease.

     Any insurance carried by Landlord, Sublessor, or Sublessee with respect to
the Premises and property therein or occurrences thereon shall, if it can be so
written without additional premium, or with an additional premium which the
requesting party agrees to pay, include a clause or endorsement denying to the
insurer right of subrogation against that party to the extent rights have been
waived by the insured prior to occurrence of injury or loss. Each party,
notwithstanding any provisions of this Sublease to the contrary, hereby waives
any rights of recovery against the other for property covered by insurance
containing such clause or endorsement to the extent of the indemnification
received thereunder.

     Notwithstanding any provision of the Master Lease or the Sublease to the
contrary, Sublessor shall remain primarily liable for all of its obligations
under the Master Lease.
<PAGE>

3.   PREMISES

     Sublessor hereby sublets to Sublessee and Sublessee hereby sublets from
Sublessor the leased premises of 19,430 rentable square feet (the "Premises"),
representing the entire 20th Floor of 475 Sansome Street, as shown on Exhibit B
from the Commencement Date of this Sublease until April 30, 2002.

4.   TERM

     The term of this sublease shall commence on the Commencement Date and end
on the 30th day of April 2002.

     Commencement Date shall occur no later than 45 days following mutual
execution of this Sublease and Landlord consent. At the option of Sublessee,
Commencement Date can occur sooner than the 45 day period if Sublessee notifies
Sublessor in writing of the date of taking occupancy.

5.   RENTAL

     Sublessee shall pay to Sublessor, without deduction, set off, prior notice
or demand, as rental, the sum of $48,575.00 per month, payable in advance in
lawful money of the United States of America on the first day of each month for
the period starting with the Commencement Date through April 2002. Should the
Commencement Date be earlier than October 1, 1997, Sublessee shall pay the
prorata rent for the period starting with the Commencement date through October
1, 1997.

     Payment for rent and any other obligations under the Sublease shall be
delivered to Sublessor's offices on or before the first day of each calendar
month. Sublessee agrees, in the event of default by Sublessee, Sublessor is
entitled to all rights and remedies as defined in paragraphs 3d and 3e of the
Master Lease to cure any such default.

6.   TENANT IMPROVEMENTS

     Sublessor delivers the Premises to Sublessee in "as-is" condition with the
exception of walls which shall be spackled and painted where necessary and the
carpets which shall be shampooed. Subleases agrees that any and all additional
and subsequent improvement work in the Premises be approved in writing by
Landlord and consented to by Sublessor; such consent will not be unreasonably
withheld or delayed.

     Sublessee shall be responsible for any and all modifications to the
existing data and voice wiring and the security access to the Premises.
Sublessor shall assist with any coordination with Sublessee's representatives
where modifications and/or additions to the existing are installed by
Sublessee.
<PAGE>

7.   BASE YEAR AND OPERATING EXPENSE ADJUSTMENT

     For purposes of this Sublease the "Base Year" shall be calendar year 1998.
Sublessee agrees to pay Sublessor, upon demand, Sublessee's prorata share of
increases in Building Operating Costs and Property Taxes over the Base Year.
Sublessee's percentage equals 6.25% of the Building square footage. Any utility
expenses passed on by Landlord for operating equipment such as HVAC outside of
business hours or normal usage as these costs pertain to the Premises shall be
passed on to Sublessee.

8.   DEPOSIT

A security deposit of $ 48,575.00 shall be provided to Sublessor by Sublessee
on or before the Commencement Date.

9.   FURNITURE

Sublessee agrees to purchase from Sublessor certain workstation and freestanding
furniture items as listed in Exhibit C for the sum of $ 70,000.00 due and
payable on the Commencement Date.

10.  GUARANTEE

Bronner Slosberg Humphrey Inc., ("Parent"), the parent corporation of Sublessee
joins in this Lease Agreement for the purpose of guaranteeing the duties and
obligations of Sublessee. En the event of a default by Sublessee under this
Sublease Agreement and such default is not cured within five (5) days of
written notice provided by Sublessor to Sublessee, Sublessor will send written
notice of default to Parent. Parent shall immediately cure such default. If such
default is not cured within five(5) days, Sublessor shall have full recourse
against Parent for all actual damages sustained by Sublessor under the terms of
this Sublease.

11.  RIGHT OF FIRST OPTION/ADDITIONAL SPACE

Sublessor is actively subleasing space on the 19th Floor and Sublessee shall
have a First Right of Refusal on the entire remaining available space on
the 19th, approximately 5,700 rentable square feet. When Sublessor has agreed in
writing with a prospective tenant ("Prospect") on terms of a sublease on said
space, Sublessor shall provide Sublessee written notice of its intent to enter
into said sublease including a document of evidence of terms and rent payments
agreed upon. From the date of this notice, Sublessee shall have five (5) working
days to respond in writing that it intends to enter into a sublease under the
same terms and conditions as agreed to by Sublessor and Prospect. Unless
Sublessor receives Sublessee's response within five (5) working days, the First
Right of Refusal shall be considered expired.

12.  OPTION TO RENEW

At the end of this sublease agreement or Master Lease ("Master Lease Expiration
Date"), Sublessee shall have no options to extend.
<PAGE>

13.  USE

     Lessee shall use the Premises for general office purposes and for no other
purpose without the prior written consent of Sublessor.

     Sublessee's business shall be established and conducted throughout the term
hereof in a first class manner. Sublessee shall not use the Premises for, or
carry on, or permit to be carried on, any offensive, noisy or dangerous trade,
business, manufacture or occupation nor permit any auction sale to be held or
conducted on or about the Premises. Sublessee shall not do or suffer anything to
be done upon the Premises which will cause structural injury to the premises or
the building of which the same form a part. The Premises shall not be overloaded
and no machinery, apparatus or other appliance shall be used or operated in or
upon the premises which will injure, vibrate or shake the Premises or the
building of which it is part. No use shall be made of the Premises which will in
any way impair the efficient operation of the sprinkler system (if any) within
the building containing the Premises. No musical instrument of any sort, or any
noise making devise will be operated or allowed upon the premises for the
purpose which will increase the existing rate of insurance upon the building in
which the Premises are located, or cause a cancellation of any insurance policy
covering the building or any part thereof.

     If any act on the part of Sublessee or use of the Premises by Sublessee
shall cause, directly or indirectly, any increase of Sublessor's insurance
expense, said additional expense shall be paid by Sublessee to Sublessor upon
demand. No such payment by Sublessee shall limit Sublessor in the exercise of
any other rights or remedies, or constitute a waiver of Sublessor's right to
require Sublessee to discontinue such act or use.

14.  ENTRY BY SUBLESSOR

     Sublessor reserves the reasonable right to inspect Sublessee's Premises
during business hours and with prior notice to Sublessee.

15.  REPAIR AND MAINTENANCE

     Sublessee shall use the Premises and any other items which are made
available by Sublessor to Sublessee during the term of this Sublease in a normal
and business-like manner. Sublessee shall return the Premises and such items at
the end of the term to Sublessor in the same condition as on the Commencement
Date, excluding normal wear and tear.

16.  ALTERATIONS

     Any and all alteration to the Premises shall be reviewed and approved by
Sublessor prior to any such alteration being performed. Such approval shall not
be unreasonably withheld or delayed. Sublessee is responsible for any Landlord-
assessed
<PAGE>

costs for alterations incurred at Sublessee's sole option, including any costs
at the end of this Sublease as mandated by Landlord.

17.  BROKERS

     Sublessor and Sublessee warrant that Cushman & Wakefield acted as
Sublessor's only broker and that TRI Commercial acted as Sublessee's only
broker. Sublessor shall provide TRI Commercial with a real estate brokerage
commission equal to $1.00 per rentable square foot per year payable on the
Commencement Date. The same schedule shall remain in effect for any additional
space Sublessee leases from Sublessor at 475 Sansome Street.

18.  CONFLICT WITH MASTER LEASE

     Any and all conflicts between the Master Lease and this Sublease shall be
resolved per the Master Lease.

19.  NOTICES OF DEFAULT

     Sublessor agrees to notify Sublessee of any event of default by Sublessee
within five (5) days of default or notice of default received by Sublessor from
Landlord. If Sublessee's default remains uncured for five (5) days from date of
notice, Sublessor shall notify Parent of such default.

20. PARKING

Unless Subleases is in default hereunder, and subject to any and all
restrictions placed upon Sublessor by Landlord in the Master Lease, Sublessee
shall be entitled to park six (6) automobiles in the Building's parking garage,
subject to all of the rules and regulations applicable to such parking as are
promulgated by Landlord and the Garage Operator (the "Operator") and to any
restrictions or regulations at any time imposed by the City and County of San
Francisco on Landlord's ability to offer such parking. The rental rate for such
parking shall be equal to the rate from time to time charged by the Operator or
Landlord to the public for comparable parking privileges in the building and
shall be paid directly by Sublessee to the Operator as directed in the
Operator's sole discretion.

21.  RIGHT TO SUBLEASE

     Pursuant to the Letter Agreement dated August 13, 1997 by and between
Mosten Management Company, Inc. ("Landlord"), Tesseract Corporation
("Sublessor"), Strategic Interactive group, Inc. ("Sublessee"), and Bronner
Slosberg Humphrey Inc. ("Parent"), Sublessee shall be entitled to sublease or
assign all or a portion of the Premises (including any additional premises
Sublessee may sublease from Sublessor at 475 Sansome Street during the term of
the Sublease) with the consent of Landlord and Sublessor, which shall not be
unreasonably withheld or delayed.
<PAGE>

22.  ADDRESS FOR NOTICES

Sublessor's address for notices:         Tesseract Corporation
                                         President's Office
                                         475 Sansome Street, 21st Floor
                                         San Francisco, Ca 94111

Sublessee's address for notices:         Strategic Interactive Group, Inc.
                                         Attn: Kathleen O'Meara, Vice-President
                                         Director of Finance & Operations
                                         The Prudential Tower
                                         800 Boylston Street
                                         Boston, MA 02199

Parent's address for notices:            Bronner Slosberg Humphrey Inc.
                                         The Prudential Tower
                                         800 Boylston Street
                                         Boston, MA 02199

Sublessor:                  Sublessee:                   Parent:

                            /s/ [ILLEGIBLE]^^            /s/ [ILLEGIBLE]^^
------------------          ------------------           -----------------
                                                              9/21/97
------------------          ------------------           -----------------

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

The undersigned, Landlord under the Master Lease, hereby consents to the
subletting of Premises on the terms and conditions contained in this sublease.
This consent shall apply only to this sublease and shall not be deemed to be a
consent to any other sublease.

Landlord:

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

-------------------------
<PAGE>

                                  EXHIBIT "C"

                          Strategic Interactive Group
                              Furniture Proposal

Everything is standard except where noted on the floor plan.

68 Steelcase Workstations
-------------------------

 .  Inside Cubes: Standard with 3 drawer file cabinet

 .  Window Cubes: Standard with 2 drawer file cabinet

10 Manager Offices: Standard Setup, black cabinet, 3 shelf bookcase
------------------

3 Corner Offices:   1. 2 oval table, 1 round table, 1-4 drawer file cabinet
----------------
                    2. 2 oval table, 1 round table, 1-4 drawer file cabinet
                    3. Wooden executive furniture, bookcase, 1-4 drawer cabinet,
                    6 chairs, 1 conference table

Reception:  Reception Desk, Sectional chairs

Training Room: 8 two drawer file cabinets, 6 round tables, 4 computer tables

3 oval tables - located north side

Computer Room: patch panels for RJ45 ethernet on 20th floor

100 Herman Miller chairs

Conference Room 20A: Conference table (16' long x 54" wide), 14 streamline
chairs

Conference Room 20B: Conference table (9' long x 42" wide), 6 streamline
chairs
<PAGE>

                                  EXHIBIT "B"

                            [DIAGRAM APPEARS HERE]
<PAGE>

[LETTERHEAD OF TRI COMMERCIAL APPEARS HERE]

August 13, 1997

Mr. Mark Terziani
Mr. Kelly Beardaley
Cushman & Wakefield of California, Inc.
555 California Street, 26/th/ Floor
San Francisco, CA 54104

RE: Letter Agreement

Dear Mark and Kelly:

Per your request, we have prepared the following Letter Agreement which will
modify the Assignment and Subletting provision of the Master Lease between
Mosten Management Company, Inc., and Tesseract Corporation as that Master Lease
pertains to the proposed sublease between Tesseract Corporation (Sublessor) and
Strategic Interactive Group (Sublessee). The change shall be incorporated into
the referenced proposed sublease.

The agreement is as follows:

Section 26(e) as found on page 28 of the Master lease dated November 22, 1991 by
and between Mosten Management Company, Inc., and Tesseract Corporation is hereby
deleted and without legal effect as it pertains to the rights and duties of
Mosten Management Company, Inc., U.B.S. Asset Management and Tesseract
Corporation in the sublease document dated August 21, 1997 between Strategic
Interactive Group (Sublessee) and Tesseract Corporation.

The sublease agreement dated 8/21/97 by and between Tesseract Corporation
(Sublessor) and Strategic Interactive Group (Sublessee) for space at 475 Sansome
Street shall include the following paragraph:

     "Pursuant to the Letter Agreement dated August 11, 1997 by and between
     Mosten Management Company, Inc. (Landlord), Tesseract Corporation
     (Tenant/Sublessor) and Strategic Interactive Group (Sublessee), Sublessee
     shall be entitled to sublease or assign all or a portion of the subleased
     premises (including any additional premises sublessee may sublease from
     Sublessor at 475 Sansome Street during the term of the sublease) with the
     consent of Mosten Management Company, Inc., and Tesseract Corporation which
     shall not be unreasonably withheld or delayed.

     It is further agreed and understood that this consent does not constitute
     Mosten Management Company, Inc.'s official and final consent to the
     proposed sublease."

<PAGE>

Mr. Mark Terzian
Mr. Kelly Beardsley
August 13, 1997
Page 2

AGREED AND ACCEPTED:

Mosten Management Company, Inc.               Strategic Interactive Group

By:/s/ [ILLEGIBLE]^^                          By: /s/ [ILLEGIBLE]^^
   ----------------------------                  ------------------------------

Dated: 8/18/97                                Dated: 8/18/97
      -------------------------                     ---------------------------

Tesseract Corporation

By: /s/ [ILLEGIBLE]^^
   -----------------------------

Dated: 8/19/97
      --------------------------

Thank you for your attention to this matter.

Sincerely,

/s/ John Chamberlain

John W. Chamberlain
Executive Vice President

JWC/mz

cc: Kathleen O'Meara
    Mr. Steven E. Humphrey
    Mr. James Adams
    Ms. Suzanne Ganser
<PAGE>

                              CONSENT TO SUBLEASE
                              -------------------

     The undersigned, MOSTEN MANAGEMENT COMPANY, INC., a Delaware corporation,
the landlord ("Landlord") under that certain Office Lease, dated November 22,
1991, (the "Master Lease") hereby consents to that certain Sublease, dated
August 11, 1997 by and between Tesseract Corporation ("Sublessor") and Strategic
Interactive Group ("Subtenant"), of the Premises described in the Master Lease.

     Nothing contained in this Consent shall or shall be deemed in any way to:

     1. Constitute a waiver, amendment or modification of any term or condition
of the Master Lease, including, without limitation, the restrictions in the
Master Lease against further assignment or subletting by the Subtenant;

     2. Create a contractual or other direct relationship between Subtenant and
Landlord;

     3. In any way limit the obligations of Sublessor or the rights of Landlord
under the Master Lease;

     4. Constitute approval or acceptance by landlordof any terms or conditions
contained in the Sublease; or

     5. Constitute Landlord's consent to any future or additional subleases of
any portion of the Premises covered by the Master Lease.

     A copy of the Sublease to which this Consent applies is attached to this
Consent as Exhibit A.
           ---------

     This consent was executed as of this 22 day of August 1997.

                              MOSTEN MANAGEMENT COMPANY, INC.
                              a Delaware corporation

                              By /s/[ILLEGIBLE]
                                 ---------------------------------------------

                              Its ____________________________________________

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