<PAGE>
                                                               Exhibit 10.20(g)

                                 SIXTH AMENDMENT
                                     TO THE
                           REVOLVING CREDIT AGREEMENT
                                 BY AND BETWEEN
       TELEPHONE AND DATA SYSTEMS, INC. AND AERIAL OPERATING COMPANY, INC.

                  This Sixth Amendment (the "SIXTH AMENDMENT") to the Revolving
Credit Agreement dated as of August 31, 1998, as amended by the First Amendment
thereto dated as of November 3, 1998, the Second Amendment thereto dated as of
February 15, 1999, the Third Amendment thereto dated as of July 22, 1999, the
Fourth Amendment thereto dated as of November 1, 1999 and the Fifth Amendment
thereto dated as of November 1, 1999 (the "REVOLVING CREDIT AGREEMENT") by and
between Telephone and Data Systems, Inc. ("TDS"), a Delaware corporation, and
Aerial Operating Company, Inc. (the "COMPANY"), a Delaware corporation, is dated
and effective as of this 10th day of March, 2000. Undefined, capitalized terms
shall have the meanings assigned to such terms in the Revolving Credit
Agreement.

                  WHEREAS, TDS and the Company are parties to the Revolving
Credit Agreement and have agreed to enter into this Sixth Amendment on the terms
and conditions set forth herein.

                  NOW, THEREFORE, in consideration of the premises set forth
above, and for good and valuable consideration, the receipt and sufficiency of
which are hereby acknowledged, and intending to be legally bound, TDS and the
Company agree to amend the Revolving Credit Agreement as follows:

1.       AMENDMENTS TO THE REVOLVING CREDIT AGREEMENT. Effective as of the date
         first above written and subject to the execution of this Sixth
         Amendment by the parties hereto, the Revolving Credit Agreement shall
         be and hereby is amended as follows:

         1.1      Schedule I to the Revolving Credit Agreement shall be replaced
                  by the new Schedule I to the Revolving Credit Agreement
                  attached to this Sixth Amendment.

         1.2      Section 5 of the Credit Agreement is hereby amended by
                  striking the first sentence thereof and replacing it with the
                  following sentence.

                  "Unless sooner terminated as elsewhere provided herein, this
                  Revolving Credit Agreement and TDS's obligation to furnish the
                  Credit shall terminate upon the earlier to occur of the
                  closing (the "Closing") of the transactions contemplated by
                  the Agreement and Plan of Reorganization dated as of September
                  17, 1999 among VoiceStream Wireless Corporation , VoiceStream
                  Wireless Holding Corporation, VoiceStream Subsidiary III
                  Corporation, the Company and TDS or December 17, 2000."


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         1.3      The definition of "Applicable Maximum Amount" set forth in
                  Section 10(b) of the Revolving Credit Agreement is hereby
                  amended and restated to read in its entirety as follows:

                                    "APPLICABLE MAXIMUM AMOUNT" shall mean, as
                           of any date of determination, the dollar amount set
                           forth in Schedule I hereto and pertaining to the
                           period during which such date occurs, MINUS (i) the
                           aggregate principal amount of all prepayments
                           required to be paid pursuant to the last sentence of
                           Section 2 after November 3, 1998, other than amounts
                           borrowed by Aerial Communications, Inc. from time to
                           time under that certain Credit Agreement dated as of
                           June 30, 1998, as it may be amended from time to time
                           (the "Nokia Credit Agreement"), by and among the
                           Guarantor, the "Lenders" and Nokia
                           Telecommunications, Inc., as "Agent" for said
                           Lenders, and (ii) the outstanding balance at any such
                           date of determination under the Nokia Credit
                           Agreement, provided that the maximum amount available
                           for borrowing by the Company and to be funded in cash
                           by TDS under this Revolving Credit Agreement shall in
                           no event exceed $315,000,000, whether or not the
                           Nokia Credit Agreement continues to be in effect and
                           whether or not amounts thereunder continue to be
                           outstanding or available to be borrowed thereunder.

2.       CONDITIONS PRECEDENT. This Sixth Amendment shall become effective as of
         the date above written, if, and only if, TDS has received duly executed
         originals of this Sixth Amendment from the Company.

3.       REPRESENTATIONS AND WARRANTIES OF THE COMPANY. The Company hereby
         represents and warrants as follows:

         3.1      This Sixth Amendment and the Revolving Credit Agreement, as
                  amended hereby, constitute legal, valid and binding
                  obligations of the Company and are enforceable against the
                  Company in accordance with their terms.

         3.2      Upon the effectiveness of this Sixth Amendment, the Company
                  hereby reaffirms all representations and warranties made in
                  the Revolving Credit Agreement, and to the extent the same are
                  not amended hereby, agrees that all such representations and
                  warranties shall be deemed to have been remade as of the date
                  of delivery of this Sixth Amendment, unless and to the extent
                  that any such representation and warranty is stated to relate
                  solely to an earlier date, in which case such representation
                  and warranty shall be true and correct as of such earlier
                  date.

4.       REFERENCE TO AND EFFECT ON THE REVOLVING CREDIT AGREEMENT.

         4.1      Upon the effectiveness of SECTION 1 hereof, on and after the
                  date hereof, each reference in the Revolving Credit Agreement
                  to "this Agreement," "hereunder," "hereof," "herein" or words
                  of like import shall mean and be a reference to the Revolving
                  Credit Agreement as amended hereby, and each reference to the
                  Revolving Credit Agreement in any other document, instrument
                  or agreement


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                  shall mean and be a reference to the Revolving Credit
                  Agreement as modified hereby.

         4.2      The Revolving Credit Agreement, as amended hereby, and all
                  other documents, instruments and agreements executed and/or
                  delivered in connection therewith, shall remain in full force
                  and effect, and are hereby ratified and confirmed.

         4.3      Except as expressly provided herein, the execution, delivery
                  and effectiveness of this Sixth Amendment shall not operate as
                  a waiver of any right, power or remedy of TDS, nor constitute
                  a waiver of any provision of the Revolving Credit Agreement or
                  any other documents, instruments and agreements executed
                  and/or delivered in connection therewith.

5.       GOVERNING LAW. This Sixth Amendment shall be governed by and construed
         in accordance with the other remaining terms of the Revolving Credit
         Agreement and the internal laws (as opposed to conflict of law
         provisions) of the State of Illinois.

6.       PARAGRAPH HEADINGS. The paragraph headings contained in this Sixth
         Amendment are and shall be without substance, meaning or content of any
         kind whatsoever and are not a part of the agreement among the parties
         hereto.

7.       COUNTERPARTS. This Sixth Amendment may be executed in one or more
         counterparts, each of which shall be deemed an original, but all of
         which together shall constitute one and the same instrument.


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<PAGE>

                  IN WITNESS WHEREOF, the parties hereto, by their duly
authorized representatives, have executed this Sixth Amendment to the Revolving
Credit Agreement, effective as of the date first written above.


TELEPHONE AND DATA SYSTEMS, INC.               AERIAL OPERATING COMPANY, INC.


By:      /S/ SANDRA L. HELTON                  By:      /S/ DONALD W. WARKENTIN
         ----------------------------------             ------------------------
Name:    Sandra L. Helton                      Name:    Donald W. Warkentin
Title:   Executive Vice President - Finance    Title:   President


                      SIGNATURE PAGE TO SIXTH AMENDMENT TO
                       REVOLVING CREDIT AGREEMENT BETWEEN
                        TDS AND AERIAL OPERATING COMPANY


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<PAGE>

                                   SCHEDULE I
                                       TO
                           REVOLVING CREDIT AGREEMENT

<TABLE>
<CAPTION>

PERIOD                                              APPLICABLE MAXIMUM AMOUNT*
------                                              --------------------------
<S>                                                 <C>
November 30, 1998 through December 30, 1998         $585,000,000
December 31, 1998 through January 30, 1999          $615,000,000
January 31, 1999 through February 14, 1999          $625,000,000
February 15, 1999 through July 21, 1999             $650,000,000
July 22, 1999 through October 31, 1999              $775,000,000
November 1, 1999 through the earlier of the
  Closing or August 31, 2000                        $355,000,000
September 1, 2000 through the earlier of the
  Closing or September 30, 2000                     $375,000,000
October 1, 2000 through the earlier of the
  Closing or October 31, 2000                       $400,000,000
November 1, 2000 through the earlier of the
  Closing or November 30, 2000                      $425,000,000
December 1, 2000 through the earlier of the
  Closing or December 17, 2000                      $460,000,000
</TABLE>

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* In no event will the amount funded by TDS exceed $315,000,000.