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                                                                   EXHIBIT 10.20

                    SIXTH AMENDMENT TO OFFICE LEASE AGREEMENT

     THIS SIXTH AMENDMENT TO OFFICE LEASE AGREEMENT (this "Sixth Amendment") is
made and entered into effective as of February 1, 2003, 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 those certain Second
and Third Amendments to Office Lease Agreement dated as of July 1, 1998, that
certain Fourth Amendment to Office Lease Agreement dated February 1, 1999, and
that certain Fifth Amendment to Office Lease Agreement dated February 28, 2002
(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 75080, which Lease is incorporated herein by reference; and

     WHEREAS, Landlord and Tenant desire to further amend the Lease to reduce
the size of the Premises and to extend the lease term.

     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, to the above Recitals, and
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 reduced to 16,330 square feet of rentable area
          by deleting 5,952 square feet of rentable area on the third floor
          south, such "Give Back Space" being described on Exhibit "A", attached
          hereto and incorporated herein for all purposes and adding 2,055
          square feet of rentable area on the second floor, such "Additional
          Space" being also described on Exhibit "A". The Additional Space is
          commonly known as Suite 2400 and was previously the "Premises" under
          that certain Office Lease Agreement dated October 14, 1999 (as amended
          -- see Section 8 below).

     3.   Basic rental for this renewal term is: $662,385.57.

     4.   The monthly rental installment is decreased effective February 1,
          2003, as set forth below:

          Monthly from February 1, 2003 through October 31, 2005 - $20,072.29.

     5.   The term of the lease is hereby extended to expire on October 31,
          2005, in lieu of April 30, 2003.

     6.   Landlord, at Landlord's sole cost and expense agrees to install an
          additional 15 tons of air conditioning on the third floor for the
          benefit of the VO lab in addition to taking responsibility for service
          and maintenance of the two (2) existing 5-ton Liebert air conditioning
          units servicing the VO lab. Furthermore, Tenant agrees that ownership
          of the two (2) existing 5-ton Liebert units rests with the Landlord.

     7.   Tenant may use two (2) covered parking spaces under the carport at no
          cost for the term described herein.

     8.   Effective February 1, 2003, subject to the full and final execution of
          this Sixth Amendment by both Landlord and Tenant, Landlord and Tenant
          mutually agree to terminate those certain Office Lease Agreements
          dated October 14, 1999 (as amended) and January 25, 2002, by and
          between Landlord and Tenant pertaining to Suites 2400 and 339
          respectively at 1701 N. Collins Blvd., Richardson, Texas 75080, and as
          such, Tenant's right to occupy said suites shall expire on
          February 1, 2003.

     9.   All notices required to be sent to the Tenant shall also be sent to
          Sonus Networks, Inc., 5 Carlisle Road, Westford, MA 01886, Attn: Chief
          Financial Officer.

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

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

     IN WITNESS WHEREOF, this Sixth 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/ G.M. Eastep
                                       --------------------------------
                                    Name:  G.M. EASTEP
                                          ------------------------------
                                    Title: Chief Operating Officer
                                          -----------------------------

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         [GRAPHIC DEPICTING 3RD LEVEL FLOOR PLAN]

                      EXHIBIT "A"
                       SITE PLAN

               telecom technologies, inc.
                  1701 N. Collins Blvd.
                Richardson, Texas 75080

Premises:         14,275 square feet of rentable area, third floor
Additional Space:  2,055 square feet of rentable area, second floor
                  -----
                  16, 330 square feet of rentable area


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      [GRAPHIC DEPICTING 2ND LEVEL FLOOR PLAN]

                   EXHIBIT "A"
                    SITE PLAN

            telecom technologies, inc.
               1701 N. Collins Blvd.
             Richardson, Texas 75080

Additional Space: 2,055 square feet of rentable area