Document:

<PAGE>

                                                                   Exhibit 10.16

                   THIRD AMENDMENT TO OFFICE LEASE AGREEMENT

     THIS THIRD AMENDMENT TO OFFICE LEASE AGREEMENT (this "Third Amendment")
is made and entered into effective as of July 1, 1998, by and between Collins
Campbell Joint Venture ("Landlord") and telecom technologies, inc. ("Tenant").

                                    RECITALS
                                    --------

     WHEREAS, Landlord and Tenant entered into that certain Office Lease
Agreement dated as of April 4, 1997, that certain First Amendment to Office
Lease Agreement, dated November 1, 1997, and that certain Second Amendment to
Office Lease Agreement dated July 1, 1998 collectively, (the "Lease"), with
respect to certain Premises defined therein and commonly known as Suite 3000 at
1701 North Collins Blvd., Richardson, Dallas County, Texas, which Lease is
incorporated herein by reference;

     WHEREAS, Landlord and Tenant desire to further amend the Lease to adjust
and clarify payment terms;

     NOW, THEREFORE, in consideration of the mutual agreements herein set
forth, the mutual agreements set forth in the Lease, and other good and
valuable consideration, the receipt and sufficiency of which are hereby
acknowledged, Landlord and Tenant have agreed, and do hereby agree, as follows:

     1.  All terms used herein and not specifically defined shall have the
         same meaning herein as is ascribed to them in the Lease.

     2.  Basic Rental is recalculated to be $3,009,171.04.

     3.  The Monthly Rental Installments are adjusted as set forth below for
         the period from and after July 1, 1998:

         Monthly from July 1, 1998 through August 31, 1998        -$33,290.27;
         Monthly from September 1, 1998 through September 30, 1998-$44,286.10;
         Monthly from October 1, 1998 through October 31, 1998    -$53,111.53;
         and
         Monthly from November 1, 1998 through October 31,2002    -$54,746.55

     4.  Prepaid Rent/Escrow Account: The Monthly Rental Installments shall be
         paid directly from the Escrow Account pursuant to the Escrow
         Agreement, Escrow No. 3488, as same has been amended from time to
         time until such time as funds in the Escrow Account are not sufficient
         to pay the required Monthly Rental Installment. At the time there are
         not sufficient funds in the Escrow Account to pay the Monthly Rental
         Installments, the Tenant shall begin paying the Monthly Rental
         Installments. The following schedule approximates when the Escrow
         Account will be depleted.

<PAGE>

         Escrow Account Balance as of July 1, 1998: $250,137.27
         Less: July 98 Rent            $33,290.27         $216,847.00
               August 98 Rent          $33,290.27         $183,556.73
               September 98 Rent       $44,286.10         $139,270.63
               October 98 Rent         $53,111.53         $ 86,159.10
               November 98 Rent        $54,746.55         $ 31,412.55
               December 98 Rent        $54,746.55        ($ 23,334.00)
         So, effective December 1, 1998, Tenant will pay Landlord $23,334 for
         the balance of December 98 rent. Then beginning the January 1, 1999
         and continuing through October 31, 2002, Tenant will pay the monthly
         rental installment of $54,746.55 at the first of each month.

<PAGE>

Third Amendment to Office Lease Agreement
Page 2 of 2

     5.  Except as herein provided to the contrary, Tenant's lease of the
         Premises from Landlord pursuant hereto shall be on the same terms and
         conditions as those specified in the Lease.

     6.  The Lease (as amended by this Third Amendment) remains in full force
         and effect and is hereby ratified and affirmed.

     7.  All other terms and conditions of the Lease shall remain unchanged
         and in full force and effect.

     IN WITNESS WHEREOF, this Third Amendment is hereby executed effective as
of the day and year first set forth above.

                                        LANDLORD:

                                        COLLINS CAMPBELL JOINT VENTURE
                                          a Texas general partnership

                                        By: Jaytex Properties, Ltd.,
                                              a Texas limited partnership

                                            By: JRS Management, Inc.

                                               By: /s/ W.T. Field
                                                  ------------------------
                                                   W.T. Field, President

                                        TENANT:

                                        telecom technologies, inc.,
                                          a Texas corporation

                                        By: /s/ Anousheh Ansari
                                           ----------------------------
                                            Anousheh Ansari, President

<PAGE>

                  [Letter head of THOMPSON REALTY CORPORATION]

                                August 26, 1998

Kevin R. Griffin
State Bank & Trust
8214 Westchester
Dallas, Texas 75225

     RE:   Escrow Agreement #3488 between Collins Campbell Joint Venture and
           telecom technologies, inc. ("TTI") with State Bank & Trust Company,
           as Escrow Agent

Dear Mr. Griffin:

     Enclosed for your file is a copy of the Third Amendment to Office Lease
Agreement effective July 1, 1998. TTI has leased an additional 11,717 rentable
square feet on the second floor of the pertinent building. Accordingly, the
lease payments have changed. Please let your records reflect that effective
September 1, 1998, and continuing through September 30, 1998, the revised
monthly rental payment to be paid out of escrow fund is $44,286.10. Then
beginning October 1, 1998, and continuing through October 31, 1998, the revised
monthly rental payment to be paid out of the escrow fund will be $53,111.53.
Then beginning November 1, 1998, and continuing through the depletion of the
escrow account, which we calculate to be on or around December 1, 1998, the
revised monthly rental payment to be paid out of the escrow fund will be
$54,746.55.

                                  Sincerely,

                                  COLLINS CAMPBELL JOINT VENTURE,
                                       a Texas joint venture

                                  By: Jaytex Properties, Ltd., joint venturer

                                       By: JRS Management, Inc., general partner

                                            By: /s/ W.T. Field
                                                -----------------------------
                                                 W.T. Field, President

ACCEPTED AND AGREED TO, as of this 8th day of September, 1998.
TELECOM TECHNOLOGIES, INC., a Texas corporation

By: Anousheh Ansari
    ------------------------------
    Anousheh Ansari, President

<PAGE>

                  [Letter head of THOMPSON REALTY CORPORATION]

                               September 9, 1998

Ms. Anousheh Ansari
President
telecom technologies, inc.
1701 North Collins Blvd.
Suite 3000
Richardson, Texas 75080

     RE:   Right of First Refusal Challenge

Dear Anousheh:

     Pursuant to Rider No. 201 of your Lease at 1701 North Collins Blvd.,
please be advised that we are considering entering into a lease with HPS
America, Inc.  The size of the lease is 1,601 rentable square feet ("RSF"). The
entire 1,601 rentable square feet is located within the Growth Area (see
Exhibit A attached) described in your Lease. The terms of the proposed lease
would be as follows:

Commencement Date:      November 1, 1998

Demised Premises:       1,601 RSF, Second Floor

Annual Rental Rate:     Year One (1) - $22.00/RSF
                        Year Two (2) - $22.00/RSF
                        Year Three (3) - $22.00/RSF
                        Year Four (4) - $22.00/RSF
                        Year Five (5) - $22.00/RSF

Expense Stop:           1998 Base Year

Space Planning and
Improvement Allowance:  $18.00/RSF

Please deliver a signed copy of this notice back to us at your earliest
convenience.

                                   Very truly yours,

                                 /s/ Douglas Thompson

                                 H. Douglas Thompson

HDT:sc

<PAGE>

Ms. Anousheh Ansari
telecom technologies, inc.
September 9, 1998
Page 2 of 2

telecom technologies, inc. has reviewed this notice and by its signature below
Accept to take the 1,601 rentable square feet.

AGREED TO AND ACCEPTED THIS 14 DAY OF SEPTEMBER, 1998.

telecom technologies, inc.

/s/ Anousheh Ansari
----------------------------
Anousheh Ansari, President

A mutually agreable start date will be negotiated

<PAGE>

                                  EXHIBIT "A"

                                   SITE PLAN

                               RPS AMERICA, INC.

                     PREMISES : 1,601 RENTABLE SQUARE FEET

                               1701 : N. COLLINS

                                   SUITE 2100

                              RICHARDSON, TX 75080

[Diagram of 2nd Level Floor Plan]<PAGE>

                                                                   Exhibit 10.17

                   FOURTH AMENDMENT TO OFFICE LEASE AGREEMENT

     THIS FOURTH AMENDMENT TO OFFICE LEASE AGREEMENT (this "Fourth
Amendment") is made and entered into effective as of February 1, 1999, by and
between Collins Campbell Joint Venture ("Landlord") and telecom technologies,
inc. ("Tenant").

                                    RECITALS
                                    --------

     WHEREAS, Landlord and Tenant entered into that certain Office Lease
Agreement dated as of April 4, 1997, as amended by that certain First Amendment
to Office Lease Agreement dated as of November 1, 1997 and by that certain
Second and Third Amendment to Office Lease Agreement dated as of July 1, 1998,
collectively (the "Lease"), with respect to certain Premises defined therein
and commonly known as Suite 3000 at 1701 North Collins Blvd., Richardson,
Dallas County, Texas, which Lease is incorporated herein by reference; and

     WHEREAS, Landlord and Tenant desire to further amend the Lease to extend
the Lease Term, increase the size of the Premises, and increase the Basic
Rental.

     NOW, THEREFORE, in consideration of the mutual agreements herein set
forth, the mutual agreements set forth in the Lease, and other good and
valuable consideration, the receipt and sufficiency of which are hereby
acknowledged, Landlord and Tenant have agreed, and do hereby agree, as follows:

     1.  All terms used herein and not specifically defined shall have the same
         meaning herein as is ascribed to them in the Lease.

     2.  The Premises are hereby increased to 38,016 square feet of rentable
         area by adding 6,072 square feet of rentable area on the second
         floor, the "Expansion Space" described on Exhibit "A", which is
         attached hereto and incorporated herein for all purposes.

     3.  The Lease Term is hereby extended to end on April 30, 2003 in lieu of
         October 31, 2002.

     4.  Basic Rental is increased to $3,909,279.04.

     5.  The "Monthly Rental Installment" is increased as set forth below:

<PAGE>

         Monthly from February 1, 1999 through October 31, 2002 - $65,878.55;
         then Monthly from November 1, 2002 through April 30, 2003 - $66,528.00.

     6.  Landlord agrees to install, in accordance with the Work Letter
         Agreement attached hereto as Exhibit "C", the improvements described
         in Exhibit "C". Landlord has made no representations as to the
         condition of the Premises or the Building, or Landlord's undertaking
         to remodel, repair or decorate, except as expressly set forth herein
         and in Exhibit "C".

     7.  Except as herein provided to the contrary, Tenant's lease of the
         Premises from Landlord pursuant hereto shall be on the same terms and
         conditions as those specified in the Lease.

     8.  The Lease (as amended by this Fourth Amendment) remains in full force
         and effect and is hereby ratified and affirmed.

     9.  Special Provisions: Exhibits "A" and "C". Exhibit "B" was
         intentionally omitted.

     10. The submission of this Lease to Tenant shall not be construed as an
         offer, nor shall Tenant have any rights with respect hereto unless and
         until Landlord shall execute a copy of this Lease and deliver the same
         to Tenant.

<PAGE>

Fourth Amendment to Office Lease Agreement
Page 2 of 2
     IN WITNESS WHEREOF, this Fourth Amendment is hereby executed effective as
of the day and year first set forth above.

                                         LANDLORD:

                                         COLLINS CAMPBELL JOINT VENTURE
                                          a Texas general partnership

                                         By: Jaytex Properties, Ltd.,
                                             a Texas limited partnership

                                             By: JRS Management, Inc.

                                         By: /s/ W.T. Field
                                         --------------------------
                                             W.T. Field, President

                                         TENANT:
                                         telecom technologies, inc.,
                                           a Texas corporation

                                         By: /s/ Anousheh Ansari
                                         --------------------------------
                                         Anousheh Ansari, President

<PAGE>

                                   Exhibit A

[Diagram of 2nd Level Floor Plan]

<PAGE>

                                  EXHIBIT "C"

                             WORK LETTER AGREEMENT

telecom technologies, inc.
1701 North Collins Blvd.
Suite 3000
Richardson, Texas 75080

RE:  Suite 2200, 1701 North Collins Blvd., The Atrium on Collins Phase II,
     Richardson, Texas.

Dear Anousheh:

     You (hereinafter referred to as "Tenant") and we (hereinafter referred to
as "Landlord") are executing, simultaneously with this "Work Letter Agreement"
(herein so called), a written Fourth Amendment to Office Lease Agreement (the
"Fourth Amendment") covering the space referred to above (hereinafter called
the "Expansion Space").

     To induce Tenant to enter into such Fourth Amendment (which is hereby
incorporated by reference) and in consideration of the mutual covenants
hereinafter contained, Landlord and Tenant mutually agree as follows:

     1.   Final Plans. Landlord agrees to provide, by Landlord's designated
     space planner, architect and/or engineer, the following Building Standard
     (hereinafter defined) space plans and architectural and mechanical
     drawings and specifications (hereinafter collectively referred to as the
     "Final Plans"), to be drawn for the Expansion Space on Tenant's behalf:

          (a)  Complete Building Standard "Space Plans" (herein so called) for
          the layout of the Expansion Space;

          (b)  Complete, finished and detailed 1/8 inch scale architectural
          drawings and specifications for Tenant's partition layout, reflected
          ceiling, telephone and electrical outlets, and finish schedule for
          the work to be done by Landlord under Paragraph 3 hereof (the
          "Construction Plans"); and

          (c)  Complete Building Standard mechanical plans and specifications
          where necessary for installation of normal air conditioning system
          and duct work and heating and electrical facilities for the work to
          be done by Landlord under Paragraph 3 hereof (the "MP & E Plans").

     Tenant shall pay all costs of preparing the Final Plans, subject, however,
to the reimbursement provisions of Paragraph 7 below.

     2.   Preparation of Final Plans; Changes to Final Plans. Tenant covenants
and agrees to furnish to Landlord all information necessary for the preparation
of each of the Space Plans, the Construction Plans and the M P & E Plans on or
before October 30, 1998. Landlord will cause the Space Plans, the Construction
Plans, and the M P & E Plans to be prepared from such information and will
submit such plans to Tenant. Within ten (10) days after receipt thereof, Tenant
shall approve the Space Plans, Construction Plans and M P & E Plans or indicate
what changes, if any, that it desires to make. Such proposed changes, if any,
shall be submitted to Landlord in writing for Landlord's written approval. If
within ten (10) days after receipt thereof, Tenant fails to approve any of the
Space Plans, Construction Plans and M P & E Plans or if Tenant fails to propose
in writing any changes to be made to such Space Plans, Construction Plans and M
P & E Plans, then Tenant shall be deemed to have approved each of same. Any
redrawing of all or any of the Final Plans occasioned by Tenant after Tenant's
approval (or deemed approval) thereof as well as any changes requested by
Tenant in connection with its initial review of the Final Plans which are
agreed to by Landlord shall be at Tenant's sole cost subject to credit in
Paragraph 7.

     3.   Construction of Improvements. Subject to each of Paragraph 2 and
Paragraph 28 of the Lease, provided that Tenant has not committed an event of
default pursuant to Paragraph 20 of the Lease, Landlord agrees to cause the
improvements to the Expansion Space to be constructed pursuant to and in
substantial accordance with the Final Plans. The construction of such
improvements in the Expansion Space shall be at Tenant's sole cost and
expense, subject, however, to the reimbursement provisions of Paragraph 7
below. Tenant acknowledges that all work done in the Expansion Space pursuant
to this Work Letter Agreement shall be performed by a contractor (and such
subcontractors, suppliers and laborers) designated by Landlord and approved
by Tenant.

     4.   Condition of Expansion Space. Landlord will, at Landlord's sole cost
and expense, construct all improvements to the Expansion Space necessary to
characterize the Expansion Space as being in Shell Condition (hereinafter
defined). As used herein the term "Shell Condition" means:

     (a)  The sprinkler system and sprinkler heads have been installed.

     (b)  The main ducting for the HVAC system has been installed.

     (c)  Electric lines have been run to junction boxes in the Expansion Space.

     (d)  The Ceiling grid has been installed. Building standard ceiling tile
          and lights stacked on the floor.

     (e)  Smooth concrete floor slab.

<PAGE>

     5.   Modifications to Shell Condition. To the extent that the Final Plans
call for any modifications (herein "Shell Modifications") to the Shell
Condition, the costs of such Shell Modifications shall be borne solely by
Tenant, Subject, however, to the reimbursement provisions of Paragraph 7 below.
By way of example, and not by way of limitation, the following shall each be
examples of Shell Modifications:

     (a)  Any modifications to the existing sprinkler system, or the moving of
     existing sprinkler heads, or the installation of additional sprinkler
     heads.

     (b)  Any modifications to the existing HVAC system, the moving of the
     existing ductwork and/or diffusers, or the installation of additional
     ductwork and/or diffusers.

     (c)  Any rewiring of existing junction boxes, relocation of existing
     junction boxes, or installation of additional junction boxes.

     6.   Building Standard. Tenant shall be required to use, and the Final
Plans shall specify Building Standard (a) light fixtures, (b) doors, (c)
ceiling tiles, and (d) hardware throughout the Expansion Space and the costs of
purchasing, transporting and installing each of the foregoing Building Standard
items shall be borne solely by Tenant, subject, however, to the reimbursement
provisions of Paragraph 7 below. Whenever the term "Building Standard" is used
in this Work Letter Agreement, it shall mean the exclusive type, brand,
quality, and/or quantity of materials Landlord designates from time to time to
be the quality or quantity to be used in the Building.

     7.   Payment of Costs; Credit. Tenant agrees to pay Landlord, promptly
upon being billed therefor, the actual cost (labor, materials, architectural,
space planning, engineering and other costs) of all work performed pursuant
to this Work Letter Agreement; Landlord shall have the right to submit
interim statements of cost incurred which shall be promptly paid by Tenant to
Landlord. Tenant agrees that all payments due to Landlord pursuant hereto
shall constitute payments of additional rent and that in the event of default
of payment thereof, Landlord shall (in addition to all other remedies) have
the same rights as in the event of default of payment of rent under the
Lease. Notwithstanding the above, so long as no event of default, as defined
in Paragraph 20 of the Lease, shall have occurred, Landlord shall credit
Tenant in an amount equal to $15.10 per square foot of Rentable Area in the
Expansion Space (the "Credit").

     8.   Delays. If Landlord shall be delayed in substantially completing the
work to be performed by Landlord pursuant to this Work Letter Agreement as a
result of:

          (a)  Tenant's failure to timely furnish information or specifications
          in accordance with Paragraph 2 above; or

          (b)  Tenant's request for materials, finishes or installations other
          than Landlord's Building Standard; or

          (c)  Tenant's changes in or modifications to any plans and
          specifications or any of the Final Plans; or

          (d)  The performance of any work in the Expansion Space by a person,
          firm or corporation employed by Tenant; (all such persons, firms or
          corporations being subject to the approval of Landlord);

<PAGE>

     Tenant's obligation for payment of rental under the Fourth Amendment (as
affected by such waiver) shall be accelerated by the number of days of such
delay.
     9.   Entry by Tenant's Agents. Landlord will permit Tenant and its agents
to enter the Expansion Space prior to the date specified for the commencement
of Tenant's occupancy under the Fourth Amendment, in order that Tenant may
perform through its own contractors (to be first approved by Landlord) such
other work and decorations as Landlord may approve at the same time that
Landlord's contractors are working in the Expansion Space. The foregoing
license to enter, however, is conditioned upon Tenant's workmen and mechanics
working in harmony and not interfering with the labor employed by Landlord,
Landlord's mechanics or contractors or by any other tenant or their
contractors. Such license is further conditioned upon workers' compensation and
public liability insurance for bodily injury and property damage, all in
amounts and with companies and on forms satisfactory to Landlord, being
provided and at all times maintained by Tenant's contractors engaged in the
performance of the work, and certificates of such insurance being furnished to
Landlord prior to proceeding with the work. If at any time such entry shall
cause disharmony or interference therewith, Landlord may withdraw this license
upon forty-eight (48) hours written notice to Tenant. Such entry conditions
shall be deemed to be under all of the terms, covenants, provisions and
conditions of the Lease. Landlord shall not be liable in any way for any
injury, loss or damage which may occur to any of Tenant's decorations or
installations, the same being solely at Tenant's risk, and Tenant shall hold
Landlord harmless from any claim, demand or action arising from activities of
Tenant's contractors, workmen or mechanics.

     If the foregoing correctly sets forth our understanding, kindly
acknowledge your approval in the space provided below whereupon this work
letter shall become a binding agreement between us.

                                        Yours very truly,

                                        COLLINS CAMPBELL JOINT VENTURE,
                                          a Texas general partnership

                                        By: Jaytex Properties, Ltd.,
                                            a Texas limited partnership

                                            By: JRS Management, Inc.,
                                                  a Texas corporation

                                                By: /s/ W. T. Field
                                                    ---------------------
                                                    W. T. Field, President

AGREED TO AND ACCEPTED
as of the 2 day of October, 1998.

telecom technologies, inc.,
  a Texas corporation

By: /s/ Anousheh Ansari
    -----------------------------
    Anousheh Ansari,
    President

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