<PAGE>
 
                                                                   EXHIBIT 10.23

                      FIRST AMENDMENT TO LEASE AGREEMENT

     This First Amendment to Lease Agreement ("First Amendment") is entered into
by and between Lee Partners, Ltd., a Texas limited partnership ("Landlord") and 
Data Race, Inc., a Texas corporation ("Tenant").

     For and in consideration of One and No/l00 Dollar ($1.00) and other good 
and valuable consideration, the receipt and sufficiency of which are 
acknowledged, Landlord and Tenant hereby recite and agree as follows:

     1.   Recitals.
          --------

          (a)  Lease Agreement.  Landlord and Tenant entered into a Lease 
               ---------------
Agreement (the "Lease") dated June 20, 1995 pertaining to two parcels of land in
Bexar County, Texas, more particularly described in the Lease.

          (b)  Amendment.  Landlord and Tenant desire to amend the Lease with 
               ---------
respect to certain obligations of Landlord and Tenant for the preparation of
plans and construction of improvements to the Leased Premises as hereinafter
provided.
          (c)  Defined Terms.  All capitalized terms used in this First 
               -------------
Amendment and not otherwise specifically defined herein shall have the same 
meaning as is ascribed to such terms in the Lease.

     2.   Preparation of Plans and Construction Conditions.  Paragraph 1(c) of
          ------------------------------------------------
the Lease is hereby modified as follows:

          (a)  Interior Improvement Plans.  Paragraph 1(c) (2) of the Lease is 
               --------------------------
hereby modified to provide that the Interior Plans as defined therein shall be
delivered by Tenant to Landlord no later than October 2, 1995. The Interior
Plans shall be (i) in accordance with the Interior Improvements Scope of Work
attached hereto as Exhibit B; (ii) substantially in accordance with the
                   ---------
Preliminary Drawings labeled "Offices for Data Race", prepared by Rehler, Vaughn
& Koone, Inc., dated August 31, 1995, and attached hereto as Exhibit C; and
                                                             ---------
(iii) in satisfactory form to permit Landlord and/or MCC to obtain all necessary
licenses, permits and approvals from all applicable governmental entities in
connection with the construction of the Interior Improvements.

          (b)  Landlord Approval of Cost and Plans.  Subparagraph 1(c)(3) of the
               -----------------------------------
Lease is hereby deleted and the following new paragraph is substituted therefor:


                                       1
<PAGE>
 
                    (3) Landlord Approval of Cost and Plans. Although the
                        -----------------------------------
               Interior Plans have not been completed as of the date of this
               First Amendment, Landlord and Tenant have mutually determined
               that the cost of the Interior Improvements will equal or exceed
               the sum of $750,000.00 ("Minimum Amount"). In addition, Tenant
               has requested a change to the Landlord's Improvements in the
               amount of $35,151.00 for the cost of additional windows as set
               forth in a memorandum from Landlord to Tenant dated July 31,
               1995. Accordingly, contemporaneously with the execution of this
               First Amendment, Tenant will deposit the sum of $785,151.00 in
               escrow with Marathon Title Company to cover the cost of
               completing the Interior Improvements and the above-referenced
               change in windows, which escrowed funds shall be held and
               disbursed pursuant to the Escrow Agreement to be entered into
               between Landlord, Tenant and Marathon Title Company in the form
               attached hereto as Exhibit A upon Tenant's delivery of such
                                  ---------
               funds. If, following the completion of the Interior Plans and the
               bidding of the construction costs pursuant to the Interior
               Construction Contract as provided in subparagraph 1(c)(4) of
               this Lease, it is determined that the cost of completing the
               Interior Improvements exceeds the Minimum Amount ("Additional
               Costs"), Tenant shall deposit the amount of such Additional Costs
               in escrow with Marathon Title Company pursuant to paragraph 
               1(d)(1) of this Lease and the Escrow Agreement.

               (c)  Waiver of Approval and Termination Rights.  Landlord and 
                    -----------------------------------------
Tenant hereby waive their respective rights to approve plans and costs and to 
terminate the Lease pursuant to subparagraphs 1(c)(3) and 1(c)(4) of the 
Lease.  Landlord and Tenant agree, however, to cooperate with each other in good
faith in the completion of the Interior Plans and the construction of the 
Interior Improvements.

               (d)  Interior Improvement Construction Contract. The first 
                    ------------------------------------------
sentence of paragraph 1(c)(4) is hereby modified to provide that the Interior
Construction Contract shall be delivered by Landlord to Tenant within fifteen
(15) days following Tenant's delivery of the Interior Plans to Landlord.

               (e)  Penalty.  Tenant acknowledges that Landlord's agreement to 
                    -------
construct Landlord's Improvements pursuant to

                                       2
<PAGE>
 
paragraph 1(a) of the Lease included a computation of cost to Landlord that was 
based on the substantial completion of the construction of improvements to 
Parcel 1 and Parcel 2 in accordance with paragraph 1 of the Lease occurring no 
later that April 30, 1996 ("Completion Date").  If Tenant fails to deliver the 
Interior Plans to Landlord by October 1, 1995, and such failure causes Landlord 
to fail to achieve substantial completion of the Landlord Improvements and 
Interior Improvements by the Completion Date, Tenant shall pay to Landlord upon 
the substantial completion of the Landlord's Improvements and the Interior 
Improvements an amount equal to $500.00 multiplied times each day beyond the 
Completion Date that the Landlord's Improvements and the Interior Improvements 
were not substantially completed due to Tenant's failure to deliver the Interior
Plans to Landlord by October 2, 1995.

     3.  Other Terms.  All other terms, conditions and provisions of the Lease 
         -----------
shall remain in full force and effect as of the date thereof.

     4.  Binding Effect.  This First Amendment shall be binding upon and insure 
         --------------
to the benefit of the parties hereto and their respective successors and 
assigns.

     EXECUTED this 1st day of September, 1995.

                                LANDLORD:

                                LEE PARTNERS, LTD., a Texas limited partnership

                                By:  LSS Company, a Texas corporation
                                                                     
                                     By:  /s/ TIMOTHY L. SWAN
                                        --------------------------------
                                        Timothy L. Swan
                                        President

                                TENANT:

                                DATA RACE, INC., a Texas corporation

                                By:  /s/ GREGORY T. SKALLA
                                   --------------------------------------
                                Name:  Gregory T. Skalla
                                     ------------------------------------
                                Title: VP Finance & CFO
                                      -----------------------------------
<PAGE>
 
                      SECOND AMENDMENT TO LEASE AGREEMENT

     This Second Amendment to Lease Agreement ("Second Amendment") is entered 
into by and between Lee Partners, Ltd., a Texas limited partnership ("Landlord")
and Data Race, Inc., a Texas corporation ("Tenant").

     For and in consideration of One and No/100 Dollar ($1.00) and other good 
and valuable consideration, the receipt and sufficiency of which are 
acknowledged, Landlord and Tenant hereby recite and agree as follows:

     1.   Recitals.
          --------

          (a)  Lease Agreement.  Landlord and Tenant entered into a Lease 
               ---------------
Agreement (the "Lease") dated June 20, 1995, as amended the 1st day of 
September, 1995, pertaining to two parcels of land in Bexar County, Texas, more 
particularly described in the Lease.

          (b)  Amendment.  Landlord and Tenant desire to amend the Lease with 
               ---------
respect to certain obligations of the Tenant for the construction of a special 
entry to the Leased Premises as hereinafter provided.

          (c)  Defined Terms.  All capitalized terms used in this Second 
               -------------
Amendment and not otherwise specifically defined herein shall have the same 
meaning as is ascribed to such terms in the Lease.

     2.   The Tenant shall have the right to construct a special entryway for 
the Parcel 1 Building, as further described in the attached Exhibit "A", subject
to the following provisions:

          (a)  The Tenant shall be solely responsible for all expenses 
associated therewith, including, but not limited to, design and construction.  
Before commencing construction, the cost of same shall be escrowed as per the 
provision of Lease paragraph 1(d)(1).

          (b)  The entryway shall be designed and constructed in such a fashion 
that it may be removed without defacing, or otherwise damaging, the original 
building.

          (c)  Upon the termination of Tenant's occupancy of the Leased 
Premises, at the sole option of the Landlord, Tenant, at its expense, shall, 
within thirty (30) days of written notice from Landlord, remove the entryway.  
Tenant shall be responsible for leaving the Building in the state which would 
have resulted if the entryway had never been constructed [see 2(b) above].

    3.    Other Terms.  All other terms, conditions and provisions of the Lease 
          -----------
shall remain in full force and effect as of the date thereof.
<PAGE>
 
     4.   Binding Effect.  This Second Amendment shall be binding upon and inure
to the benefit of the parties hereto and their respective successors and 
assigns.

     EXECUTED this 11 day of November, 1995

                                LANDLORD:

                                LEE PARTNERS, LTD., A Texas limited partnership

                                By:   /s/ TIMOTHY L. SWAN
                                   ---------------------------------
                                   Timothy L. Swan
                                   President of the General Partner

                                TENANT: 

                                DATA RACE, INC., a Texas corporation

                                By:  /s/ G. T. SKALLA
                                   ----------------------------------
                                Name:  G. T. Skalla
                                     --------------------------------
                                Title:  CFO
                                      -------------------------------

<PAGE>
 
                      THIRD AMENDMENT TO LEASE AGREEMENT

     This Third Amendment to Lease Agreement ("Third Amendment") is entered into
by and between Lee Partners, Ltd., a Texas limited partnership ("Landlord") and 
Date Race, Inc., a Texas corporation ("Tenant").

     For and in consideration of One and No/100 Dollar ($1.00) and other good 
and valuable consideration, the receipt and sufficiency of which are 
acknowledged, Landlord and Tenant hereby recite and agree as follows:

     1.   Recitals.
          --------

          (a)  Lease Agreement.  Landlord and Tenant entered into a Lease 
               ---------------
Agreement (the "Lease") dated June 20, 1995, as amended the 1st day of 
September, 1995 and the 8th day of November, 1995, pertaining to two parcels of 
land in Bexar County, Texas, more particularly described in the Lease.

          (b)  Amendment.  Landlord and Tenant desire to amend the Lease with 
               ---------
respect to certain modifications of two restrooms as further described 
hereafter.

          (c)  Defined Terms.  All capitalized terms used in this third 
               -------------
Amendment and not otherwise specifically defined herein shall have the same 
meaning as is ascribed to such terms in the Lease.

     2.   Improvement Modifications.  The Landlord agrees not to install the 
          -------------------------
toilets, urinals and toilet partitions in Restrooms 124 and 125 in Building A.  
At the sole option of Landlord and within thirty (30) days of Landlord's written
notice, upon termination of Tenant's occupancy of the Leased Premises, Tenant 
shall install the toilets, urinals and toilet partitions in Restrooms 124 and 
125, at its sole expense, in accordance with the Plans and Specifications for 
Interior Improvements signed by Landlord and Tenant, dated December 12, 1995.  
If the Tenant is required to so replace the fixtures, Tenant shall also be 
responsible for leaving the balance of the restroom improvements in an overall 
good condition similar to what would be expected had the facilities been used 
during the Lease Term for the intended restroom purpose, subject only to 
ordinary wear and tear.

     3.   Other Terms.  All other terms, conditions and provisions of the Lease 
          -----------
shall remain in full force and effect as of the date thereof.

     4.   Binding Effect.  This Third Amendment shall be binding upon and inure 
          --------------
to the benefit of the parties hereto and their respective successors and 
assigns.

     EXECUTED this 16th day of January, 1996

                                LANDLORD:

                                LEE PARTNERS, LTD., a Texas limited partnership

                                By:  /s/ TIMOTHY L. SWAN
                                   ---------------------------------
                                   Timothy L. Swan
                                   President of the General Partner

                                TENANT:

                                DATA RACE, INC., a Texas corporation

                                By:  /s/ G. T. SKALLA
                                   ---------------------------------
                                Name:  G. T. Skalla
                                     -------------------------------
                                Title:  CFO
                                      ------------------------------
                                
<PAGE>
 
                      FOURTH AMENDMENT TO LEASE AGREEMENT

     This Fourth Amendment to Lease Agreement ("Fourth Amendment") is entered 
into by and between Lee Partners, Ltd., a Texas limited partnership ("Landlord")
and Data Race, Inc., a Texas corporation ("Tenant").

     For and in consideration of One and No/100 Dollar ($1.00) and other good 
and valuable consideration, the receipt and sufficiency of which are 
acknowledged, Landlord and Tenant hereby recite and agree as follows:

     1.   Recitals.
          --------

          (a)  Lease Agreement.  Landlord and Tenant entered into a Lease 
               ---------------
Agreement (the "Lease") dated June 20, 1995, as amended the 1st day of
September, 1995, the 8th day of November, 1995 and the 16th day of January,
1996, pertaining to two parcels of land in Bexar County, Texas, more
particularly described in the Lease.

          (b)  Amendment.  Landlord and Tenant desire to amend the Lease with 
               ---------
respect to certain modifications of floor covering in a limited area as further 
described hereafter.

          (c)  Defined Terms.  All Capitalized terms used in this Fourth 
               -------------
Amendment and not otherwise specifically defined herein shall have the same 
meaning as is ascribed to such terms in the Lease.

     2.   Improvement Modifications.  The Landlord agrees not to install the 
          -------------------------
floor covering specified in the Plans and Specifications for Interior 
Improvements signed by Landlord and Tenant, dated December 12, 1995 in rooms 
241, 243, 257 and 259 in Building B.  At the sole option of Landlord and within
thirty (30) days of Landlord's written notice, upon the termination of Tenant's 
occupancy of the Leased Premises, Tenant shall install the originally specified 
floor covering, at its sole expense, in accordance with the Plans and 
Specifications for Interior Improvements signed by Landlord and Tenant, dated 
December 12, 1995.

     3.   Other Terms.  All other terms, conditions and provisions of the Lease 
          -----------
shall remain in full force and effect as of the date thereof.

     4.   Binding Effect.  This Fourth Amendment shall be binding upon and inure
          --------------
to the benefit of the parties hereto and their respective successors and 
assigns.

     EXECUTED this 20th day of February, 1996.

                                LANDLORD:

                                LEE PARTNERS, LTD., a Texas limited partnership

                                By:  /s/ TIMOTHY L. SWAN
                                   --------------------------------
                                   Timothy L. Swan
                                   President of the General Partner

                                TENANT:

                                DATA RACE, INC., a Texas corporation

                                By:  /s/ G. T. SKALLA
                                   --------------------------------
                                Name:  G. T. Skalla
                                     ------------------------------
                                Title:   CFO
                                      -----------------------------


<PAGE>
 
                      FIFTH AMENDMENT TO LEASE AGREEMENT

     This Fifth Amendment to Lease Agreement ("Fifth Amendment") is entered into
by and between Lee Partners, Ltd., a Texas limited partnership ("Landlord") and 
Data Race, Inc., a Texas corporation ("Tenant").

     For and in consideration of One and No/100 Dollar ($1.00) and other good
and valuable consideration, the receipt and sufficiency of which are
acknowledged, Landlord and Tenant hereby recite and agree as follows:

     1.   Recitals.
          --------

          (a)  Lease Agreement.  Landlord and Tenant entered into a Lease 
               ---------------
Agreement (the "Lease") dated June 20, 1995, as amended the 1st day of 
September, 1995, the 8th of November, 1995, the 16th day of January and the 20th
day of February, 1996, pertaining to two parcels of land in Bexar County, Texas,
more particularly described in the Lease.

          (b)  Amendment.  Landlord and Tenant desire to amend the Lease with 
               ---------
respect to signage and fencing installation as further described hereafter.

          (c)  Defined Terms.  All capitalized terms used in this Fifth 
               -------------
Amendment and not otherwise specifically defined herein shall have the same 
meaning as is ascribed to such terms in the Lease.

     2.   Signage.  The Landlord consents to Tenant's installation, at its sole 
          -------
expense, of signage on the western face of the entryway canopy to Building B
generally to consist of the words DATA RACE, with the entire text to cover an
area approximately 22" by 17', as further illustrated on Exhibit A hereto. Upon
the termination of Tenant's occupancy of the Leased Premises, Tenant shall, at
its sole expense, remove said signage and restore the canopy face to its
original condition as though no sign had been installed thereon.

     3.   Fencing.  The Landlord consents to Tenant's installation, at its sole 
          -------
expense, of 6' high chain link fencing to enclose the concrete drive on the
north side of Building A with two gated entrances generally as shown on Exhibit
B hereto. The requested installation violates a 15' set back from the front of
the building and Landlord has obtained an Architectural Committee approval of
the proposed installation. However, Landlord assumes no other responsibility for
the installation.

     4.   Other Terms.  All other terms, conditions and provision of the Lease
          -----------
shall remain in full force and effect as of the date thereof.

     5.   Binding Effect.  This Fourth Amendment shall be binding upon and 
          --------------
inure to the benefit of the parties hereto and their respective successors and 
assigns.

     EXECUTED this 21 day of March, 1996.

                                LANDLORD:

                                LEE PARTNERS, LTD., a Texas limited partnership

                                By:  /s/ TIMOTHY L. SWAN
                                   --------------------------------
                                   Timothy L. Swan
                                   President of the General Partner

                                TENANT:

                                DATA RACE, INC.,  a Texas corporation

                                By: /s/  G.T. SKALLA
                                    -------------------------------
                                Name:  G. T.Skalla
                                     ------------------------------
                                Title:   CFO
                                      -----------------------------