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                                                                   EXHIBIT 10.20

                                 AMENDMENT NO. 4
                                       TO
                        DMS-MTX CELLULAR SUPPLY AGREEMENT
                                     BETWEEN
                                 ALAMOSA PCS LLC
                                       AND
                              NORTEL NETWORKS INC.

        This Amendment No. 4 is made effective as of the 8th day of February,
2000, by and between Nortel Networks Inc. (successor in interest to Northern
Telecom Inc.) ("Seller") and Alamosa PCS LLC ("Buyer").

        WHEREAS, Buyer and Seller entered into a Supply Agreement dated December
21, 1998, as amended, for the sale and purchase of Seller's Equipment and
Services ("Agreement"); and

        WHEREAS, Buyer and Seller now wish to, among other things, amend the
Agreement to extend the Term and adjust Buyer's Volume Commitment; and

        NOW, THEREFORE, in consideration of the mutual covenants herein
contained, Buyer and Seller hereby agree to amend the Agreement as follows:

        1.  Amend Article 1 (Definitions), Section 1.46 by deleting the phrase
            "three years therefrom" and replacing it with "on July 1, 2002."

        2.  Amend Section 5.3 by deleting the reference to "eighty-two million
            dollars ($82,000,000) and replacing it with "one hundred sixty-seven
            million dollars ($167,000,000)."

        3.  Amend Section 5.4 by deleting the section in its entirety and
            replacing it with the following:

            "5.4  Until Buyer's total purchases under this Agreement exceed
                  eighty-two million dollars ($82,000,000) net Price in
                  Equipment and Services, such Equipment and Services purchased
                  and financed by Seller shall be subject to an additional three
                  percent (3%) financing premium ("Financing Premium") over the
                  Additional Affiliate pricing as set forth in Annex 1. For
                  subsequent purchases (i.e., purchases of Equipment and
                  Services in excess of eighty-two million dollars [$82,000,000]
                  net Price), such Financing Premium shall no longer apply."

        4.  Except as specifically modified herein, the Agreement in all other
            respects shall continue in full force and effect.

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        IN WITNESS WHEREOF, the parties hereto have caused this Amendment No. 4
to be executed by their representatives being thereunto duly authorized.


ALAMOSA PCS LLC                              NORTEL NETWORKS INC.


By:      /s/ DAVID SHARBUTT                  By:      /s/ ROBERT D. BEITER
   --------------------------------             --------------------------------
Name:    David Sharbutt
     ------------------------------          Name:    Robert D. Beiter
               (Type/Print)                       ------------------------------
                                                            (Type/Print)
Title:   Chief Executive Officer
      -----------------------------          Title:   Director, Customer Finance
Date:    February 8, 2000                          -----------------------------
     ------------------------------          Date:    February 8, 2000
                                                  ------------------------------