<PAGE>   1

                                                                   Exhibit 10.21


                       ASSIGNMENT AND ASSUMPTION AGREEMENT

        ASSIGNMENT AND ASSUMPTION AGREEMENT, dated as of July 24, 2000 (the
"Agreement"), by and between Western Wireless Corporation, a Washington
corporation ("WWC"), and VoiceStream Wireless Corporation, a Delaware
corporation ("VoiceStream", WWC and VoiceStream are sometimes individually
referred to herein as a "Party" and collectively as the "Parties").

        WHEREAS, WWC and VoiceStream are parties to that certain Agreement and
Plan of Distribution, dated as of April 9, 1999, pursuant to which, among other
things, WWC has agreed, upon the terms and conditions set forth therein, to
distribute the shares of VoiceStream's Common Stock, no par value (the "Common
Stock"), owned by it, which shares represent 80.1% of the issued and outstanding
shares of Common Stock, to WWC's shareholders on the basis of one share of
Common Stock for each one share of WWC's outstanding common stock (the "Spin
Off");

        WHEREAS, WWC and John W. Stanton (the "Employee") have entered into an
Employment Agreement, dated as of March 12, 1996 (the "Employment Agreement"),
setting forth, among other things, the terms of the Employee's employment by
WWC; and

        WHEREAS, effective as of July 24, 2000 (the "Assignment Date"), WWC
desires to assign to VoiceStream, and VoiceStream desires to assume, all of
WWC's rights and obligations under the Employment Agreement on the terms set
forth herein.

        NOW, THEREFORE, in consideration of the mutual covenants herein
contained and for other good and valuable consideration, the receipt and
sufficiency of which is hereby acknowledged, WWC and VoiceStream agree as
follows:

        1.      Assignment and Assumption of the Employment Agreement. On and as
of the Assignment Date, WWC hereby transfers, assigns and conveys to VoiceStream
all of WWC's rights and interests in, and VoiceStream hereby accepts, assumes
and agrees to perform and discharge all of the obligations arising from and
after the Assignment Date under, the Employment Agreement. All references in the
Employment Agreement to WWC shall be changed to VoiceStream; provided , that (a)
the WWC Indemnification Agreement referenced in Paragraph 6 of the Employment
Agreement shall continue to remain in effect, to the extent applicable, and (b)
the confidentiality provisions in Paragraph 8 of the Employment Agreement shall
continue to apply to WWC and its subsidiaries and the Employee.

        2.      (a) Indemnification of WWC by VoiceStream. VoiceStream shall
indemnify and hold harmless WWC, its subsidiaries and affiliates and its and
their respective shareholders, members, managers, officers, employees, agents
and/or the legal representatives of any of them (each, a "WWC Indemnified
Party"), from and against any and all demands, claims, losses, liabilities,
actions or causes of action, assessments, damages, fines, fees, penalties, costs
and


                                      -1-


<PAGE>   2
expenses, including, without limitation, interest, expenses of investigation,
reasonable fees and disbursements of counsel, accountants and other experts
(collectively, "Losses") (whether arising out of or relating to matters asserted
by third parties against a WWC Indemnified Party or incurred or sustained by
such WWC Indemnified Party in the absence of a third party claim) that arise
after the Assignment Date (a) under the Employment Agreement, (b) by reason of
the Employee's employment by VoiceStream after the Assignment Date, or (c) any
default by VoiceStream in the performance of its obligations under the
Employment Agreement.

               (b) Indemnification of VoiceStream by WWC. WWC shall indemnify
and hold harmless VoiceStream, its subsidiaries and affiliates and its and their
respective shareholders, members, managers, officers, employees, agents and/or
the legal representatives of any of them (each, a "VoiceStream Indemnified
Party"), from and against any and all Losses (whether arising out of or relating
to matters asserted by third parties against a VoiceStream Indemnified Party or
incurred or sustained by such VoiceStream Indemnified Party in the absence of a
third party claim) that arise out of or result from (a) the assignment to
VoiceStream of the Employment Agreement, (b) the Employee's employment by WWC on
or prior to the Assignment Date, or (c) any default by WWC prior to the
Assignment Date in the performance of its obligations under the Employment
Agreement.

               (c) Procedures.

                      (i) In the event that any VoiceStream Indemnified Party or
WWC Indemnified Party (each an "Indemnified Party") shall sustain or incur any
Losses in respect of which indemnification may be sought by such Indemnified
Party pursuant to this Section 2, the Indemnified Party shall assert a claim for
indemnification by giving prompt notice to the applicable indemnifying party
(the "Indemnifying Party) under Section 2 and shall thereafter keep the
Indemnifying Party reasonably informed with respect thereto; provided that
failure of the Indemnified Party to give the Indemnifying Party notice as
provided herein shall not relieve the Indemnifying Party of any of its
obligations hereunder, except to the extent that the Indemnifying Party is
materially prejudiced by such failure. Upon the Indemnifying Party's receipt of
such notice, the Indemnifying Party shall have the right to assume, conduct and
control the defense, compromise or settlement thereof, by written notice to the
Indemnified Party of its intention to do so within thirty (30) days after
receipt of the notice, with counsel reasonably satisfactory to the Indemnified
Party, at the Indemnifying Party's own expense, and thereupon to prosecute in
the name and on behalf of the Indemnified Party any available cross-claims,
counter-claims or third-party claims arising with respect to the claim. If the
Indemnifying Party shall assume the defense of such claim, it shall not settle
such claim unless such settlement includes as an unconditional term thereof the
giving by the claimant or the plaintiff of a release of the Indemnified Party,
reasonably satisfactory to the Indemnified Party, from all liability with
respect to such claim. As long as the Indemnifying Party is contesting any such
claim in good faith and on a timely basis, the Indemnified Party shall not pay
or settle any such claim. Notwithstanding the assumption by the Indemnifying
Party of the defense of any claim as provided in this Section 2(c) and without
limiting the Indemnifying Party's right to assume, conduct and control the
defense, compromise


                                      -2-


<PAGE>   3
or settlement thereof, the Indemnified Party shall be permitted to join in the
defense of such claim and to employ counsel at its own expense. Assumption by
the Indemnifying Party of the defense of any claim shall not be deemed a
concession by the Indemnifying Party that it is required to indemnify the
Indemnified Party for the subject matter of such claim.

                      (ii) If the Indemnifying Party shall fail to notify the
Indemnified Party of its desire to assume the defense of such claim within the
prescribed 30-day period set forth in Section 2(c)(i) or shall notify the
Indemnified Party that it will not assume the defense of any such claim, then
the Indemnified Party may defend any such claim, in which event it may do so in
such manner as it may deem appropriate, and the Indemnifying Party shall be
bound by any determinations made in any litigation with respect to such claim or
any settlement thereof effected by the Indemnified Party, provided that any such
determinations or settlement shall not affect the right of the Indemnifying
Party to dispute the Indemnified Party's claim for indemnification. Unless and
until the Indemnified Party assumes the defense of any claim, the Indemnifying
Party shall advance to the Indemnified Party any of its reasonable attorneys'
fees and other costs and expenses incurred in connection with the defense of any
such action or proceeding.

        3.      Representations and Warranties.

        Each of WWC and VoiceStream hereby represents and warrants to the other
as follows:

               (a) It is a corporation duly organized, validly existing and in
good standing under the laws of its jurisdiction of incorporation with full
power and authority to execute, deliver and perform its obligations under this
Agreement;

               (b) The execution and delivery of this Agreement and the
consummation of the transactions contemplated hereby have been duly and validly
authorized by all necessary corporate action on its part and this Agreement
constitutes its legal, valid and binding obligation enforceable against it in
accordance with its terms; and

               (c) Neither the execution, delivery or performance of this
Agreement by it, nor the consummation of the transactions contemplated hereby
will, with or without the giving of notice of passage of time, or both, conflict
with, result in a default or loss of rights (or give rise to any right of
termination, cancellation or acceleration) under, (i) any provision of its (or
any of its subsidiaries') certificate of incorporation or by-laws, (ii) any
material note, bond, indenture, mortgage, deed of trust, contract, agreement,
lease or other instrument or obligation to which it, or any of its subsidiaries,
is a party or by which it, or any of its subsidiaries, or its, or any of its
subsidiaries', properties may be bound or affected, or (iii) any law, order,
judgment, ordinance, rule, regulation or decree to which it, or any of its
subsidiaries, is a party or by which it, or any of its subsidiaries, or any of
its, or any of its subsidiaries', properties are bound or affected.

        4.      Effectiveness. This Agreement shall become effective upon the
Assignment Date.


                                      -3-


<PAGE>   4
        5.      Notices. All notices, claims or other communications hereunder
shall be in writing and shall be given by delivery in person, by facsimile
transmission, by registered or certified mail (return receipt requested),
postage prepaid, or overnight carrier guaranteeing next day delivery:

               (a)    to VoiceStream:

                      3650 131 Avenue SE
                      Bellevue, Washington 98006
                      Attention: General Counsel
                      Tel: (425) 586-8014
                      Fax: (425) 586-8080

                      and:

                      Friedman Kaplan & Seiler LLP
                      875 Third Avenue
                      New York, New York 10022-6225
                      Attention:  Barry A. Adelman, Esq.
                      Telephone:  (212) 833-1107
                      Facsimile: (212)-355-6401

               (b)    to WWC:

                      3650 131 Avenue SE
                      Bellevue, Washington 98006
                      Attention: General Counsel
                      Tel: (425) 586-8014
                      Fax: (425) 586-8080

                      and:

                      Friedman Kaplan & Seiler LLP
                      875 Third Avenue
                      New York, New York 10022-6225
                      Attention:  Barry A. Adelman, Esq.
                      Telephone:  (212) 833-1107
                      Facsimile: (212)-355-6401

or to such other address as any Party may from time to time furnish to the other
by a notice given in accordance with the provisions of this Section 5. All such
notices and communications shall be deemed to have been duly given at the time
delivered by hand, if personally delivered; five (5)


                                      -4-


<PAGE>   5
business days after being deposited in the mail, first class postage prepaid,
return receipt requested, if mailed; when receipt confirmed, if sent by
facsimile; and the next business day after timely delivery to the courier, if
sent by an overnight air courier service guaranteeing next day delivery.

        6.      Waiver; Amendments. This Agreement may be amended, modified or
supplemental only by written agreement of the Parties. No provision in this
Agreement shall be deemed waived except by an instrument in writing signed by
the Party waiving such provision. Any waiver by any Party of any of its rights
under this Agreement or of any breach of this Agreement shall not constitute a
waiver of any other rights or of any other or future breach.

        7.      Cumulative Remedies. The rights and remedies provided by this
Agreement are cumulative and the use of any one right or remedy by any Party
shall not preclude or waive its right to use any or all other remedies except as
otherwise expressly provided in this Agreement. Such rights and remedies are
given in addition to any other rights the Parties may have by law, statute,
ordinance or otherwise.

        8.      Successors, Assigns and Transferees. This Agreement shall inure
to the benefit of, and be binding upon, the successors and assigns of each
Party.

        9.      Entire Agreement. This Agreement constitutes the entire
understanding of the Parties with respect to the subject matter hereof and
supersedes all prior understandings among the Parties with respect to such
subject matter.

        10.     Execution in Counterparts. This Agreement may be executed in one
or more counterparts, each of which shall be deemed to be an original, and all
of which together shall be considered the same agreement.

        11.     Applicable Law. This Agreement will be governed by and construed
and interpreted in accordance with the laws of the State of Delaware without
reference to conflicts of laws principles.

        12.     Section Headings. The headings of the sections and subsections
of this Agreement are inserted for convenience only and shall not be deemed to
constitute a part thereof.

        13.     Severability. The invalidity or unenforceability of any
particular provision of this Agreement shall not affect the other provisions
hereof, and this Agreement shall be construed in all respects as if such invalid
or unenforceable provisions were omitted.


                                      -5-


<PAGE>   6
        IN WITNESS WHEREOF, the parties hereto have executed this Agreement as
of the date first above written.


                         WESTERN WIRELESS CORPORATION

                         By:
                            ------------------------------------
                         Name:
                              ----------------------------------
                         Title:
                               ---------------------------------


                         VOICESTREAM WIRELESS CORPORATION

                         By:
                            ------------------------------------
                         Name:
                              ----------------------------------
                         Title:
                               ---------------------------------


        John W. Stanton hereby accepts and acknowledges the assignment and
assumption of his Employment Agreement, effective as of July 24, 2000.


ACCEPTED AND ACKNOWLEDGED:

By:
   -------------------------------
        John W. Stanton


                                      -6-