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                                                                    EXHIBIT 99.9

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                               INVESTOR AGREEMENT

                         DATED AS OF SEPTEMBER 17, 1999

                                  BY AND AMONG

                                   SONERA LTD.
                      A FINNISH LIMITED LIABILITY COMPANY,

                        VOICESTREAM WIRELESS CORPORATION,
                            A WASHINGTON CORPORATION

                                       AND

                    VOICESTREAM WIRELESS HOLDING CORPORATION,
                             A DELAWARE CORPORATION.

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                               INVESTOR AGREEMENT

        This INVESTOR AGREEMENT (this "AGREEMENT") is made as of September 17,
1999 by and between Sonera Ltd., a limited liability company organized under the
Laws of the Republic of Finland ("Sonera"), VoiceStream Wireless Corporation, a
Washington corporation ("VOICESTREAM"), and VoiceStream Wireless Holding
Corporation, a Delaware corporation (the "COMPANY").

        WHEREAS, Aerial Communications, Inc, a Delaware corporation ("AERIAL"),
Telephone and Data Systems, Inc. ("TDS"), VoiceStream, the Company and
VoiceStream Subsidiary III Corporation, a Delaware corporation and wholly owned
subsidiary of the Company ("SUB"), have entered into an Agreement and Plan of
Reorganization, dated as of September 17, 1999 (the "AERIAL REORGANIZATION
AGREEMENT"), pursuant to which Sub will be merged into Aerial.

        WHEREAS, Voice Stream, the Company and Omnipoint Corporation, a Delaware
corporation ("Omnipoint"), have entered into an Agreement and Plan of
Reorganization, dated June 23, 1999 (the "Omnipoint Reorganization Agreement"),
whereby, among other things, a subsidiary of the Company will merge with and
into Omnipoint (the "Omnipoint Reorganization").

        WHEREAS, the Investor and the Company wish to set forth certain
agreements concerning the ownership and transfer of shares of Common Stock, and
certain other matters as provided herein.

        NOW, THEREFORE, in consideration of the mutual and dependent promises
set forth herein, the Investor hereby agrees with the Company, and the Company
hereby agrees with the Investor, as follows:

1.      EFFECTIVE DATE OF AGREEMENT AND OTHER MATTERS.

        (a) This Agreement shall become effective at the earlier of (i) the
Effective Time pursuant to and as defined in the Omnipoint Reorganization
Agreement, or (ii) the Effective Time pursuant to and as defined in the Aerial
Reorganization Agreement.

        (b) All representations, warranties and covenants made by either
VoiceStream or the Company are hereby made on a joint and several basis by
VoiceStream and the Company.

2.      DEFINITIONS.

        (a) Unless the context requires otherwise, capitalized terms used but
not defined in this Agreement have the meanings given in the Aerial
Reorganization Agreement.


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        (b) As used in this Agreement, the following terms have the respective
meanings set forth below (applicable to both the singular and plural forms of
such terms):

        "AFFILIATE" shall have the meaning set forth in Rule 12b-2 of the rules
and regulations promulgated under the Exchange Act; provided, however, that for
purposes of Section 3 of this Agreement, none of the following shall be deemed
to be an Affiliate of the Investor: (A)(i) the Company, (ii) the Cook Inlet
Parties, or (iii) any of the Current Principal Stockholders or their Affiliates,
or (B) any Person who would be an Affiliate of Investor solely because such
Person is an Affiliate of any of the Persons referred to in clause (A) of this
provision.

        "AGREEMENT" means this Investor Agreement, as amended, modified,
supplemented or restated from time to time in accordance with the terms hereof.

        "BANKRUPTCY EVENT" means the filing by the Company or VoiceStream of a
petition in bankruptcy or the expiration of sixty (60) days after a petition in
bankruptcy shall have been filed against the Company or VoiceStream and such
petition shall not have been stayed or discharged during such sixty (60) day
period; or upon the expiration of sixty (60) days after the commencement of any
proceeding under any law for the relief of debtors seeking the relief or
readjustment of the Company's or VoiceStream's indebtedness either through
reorganization, winding-up, extension or otherwise, and such proceedings
involving the Company or VoiceStream as debtor shall not have been vacated or
stayed within such sixty (60) day period; or upon the appointment of a receiver,
custodian or trustee for all or substantially all of the Company's or
VoiceStream's property, or the making by the Company or VoiceStream of any
general assignment for the benefit of creditors, or the admitting in writing by
the Company or VoiceStream of its inability to pay its debts as they mature; or
upon the voluntary or involuntary liquidation or dissolution of the Company or
VoiceStream, other than the liquidation or dissolution of VoiceStream with or
into the Company or another wholly-owned subsidiary of the Company.

        "BENEFICIALLY OWNED" and "BENEFICIAL OWNERSHIP" have the meaning set
forth in Rule 13d-3 of the rules and regulations promulgated under the Exchange
Act, excluding paragraph (d) of such Rule relating to the deemed beneficial
ownership of a security if such person has the right to acquire such security
within sixty (60) days.

        "BOARD" means the board of directors of the Company.




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        "CHANGE OF CONTROL" means the acquisition by any Person or 13D Group of
direct or indirect Beneficial Ownership of Voting Securities representing 50% or
more of the Voting Power, pursuant to (i) an acquisition of Common Stock, (ii)
any merger, consolidation or business combination involving the Company or any
material portion of its business, or (iii) a recapitalization, restructuring,
liquidation, dissolution or similar extraordinary transaction relating to the
Company or any material portion of its business; provided, that the foregoing
shall not be deemed to constitute a Change of Control if immediately after such
transaction or event Persons who were stockholders of the Company immediately
prior to such transaction or event continue to Beneficially Own on a
proportionate basis Voting Securities which represent more than fifty percent
(50%) of the Voting Power of the surviving or resulting entity immediately after
such transaction or event; provided further, that, notwithstanding the
foregoing, a Change of Control shall be deemed to occur if any one of the
Current Principal Stockholders shall Beneficially Own fifty percent (50%) or
more of the Voting Power.

        "COMMISSION" means the United States Securities and Exchange Commission.

        "COMMON STOCK" means the Company's Common Stock, $0.001, and shall
include any new, substituted and additional securities issued at any time in
replacement of the Common Stock or issued or delivered with respect to the
Common Stock.

        "COMPANY" means VoiceStream Wireless Holding Corporation, a Delaware
corporation, and its successors and assigns.

        "COOK INLET PARTIES" means (i) Cook Inlet Western Wireless PV/SS PCS,
L.P., (ii) Cook Inlet VoiceStream PCS, LLC, (iii) Cook Inlet VoiceStream II,
LLC, (iv) Cook Inlet VoiceStream III, LLC and any similar joint venture in which
the Company, Omnipoint, VoiceStream or their Subsidiaries from time to time are
significant equity owners.

        "CURRENT PRINCIPAL STOCKHOLDERS" means the individuals and entities
listed on Schedule I and their Permitted Affiliated Transferees (as defined in
the Voting Agreement) and, with respect to Hutchinson USA, its Disqualified
Affiliates (as defined in the Hutchinson USA Investor Agreement).

        "DISINTERESTED BOARD APPROVAL" means the affirmative vote or written
consent of a majority of the Board (excluding Investor Directors) then in
office.

        "EXCHANGE ACT" means the Securities Exchange Act of 1934, as amended.

        "IMPUTED COST OF FUNDS" means LIBOR plus 275 basis points.

        "INCREMENTAL SHARES" has the meaning given in Section 3(c)(i).

        "INVESTOR" unless otherwise specified herein, means Sonera and its
successors and assigns and Affiliates which from time to time hold shares of
Voting Securities.




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        "INVESTOR DIRECTOR" means any member of the Board who has been
designated by Investor or any member of the Investor Group for nomination or
appointment as a director of the Company.

        "INVESTOR GROUP" means Investor and any of their respective Affiliates.

        "INVESTOR'S OWNERSHIP PERCENTAGE" means the percentage determined by
dividing the shares of Common Stock Beneficially Owned by the Investor Group by
the Voting Power of the Company both determined as of the same relevant date of
determination.

        "INVESTOR TENDER OFFER" means a bona fide public tender offer subject to
the provisions of Regulation 14D when first commenced within the meaning of Rule
14d-2(a) of the rules and regulations under the Exchange Act, by any of the
Investor Group (or any 13D Group that includes any of the Investor Group) to
purchase or exchange for cash or other consideration any Common Stock and which
consists of an offer to acquire one hundred percent (100%) of the outstanding
Common Stock (without regard to Common Stock owned by any of the Investor Group)
and is conditioned (which condition may not be waived) on a majority of the
shares of outstanding Common Stock held by shareholders other than any of the
Investor Group being tendered and not withdrawn with respect to such offer.

        "HUTCHINSON USA" means Hutchinson Telecommunications PCS (USA) Limited,
a British Virgin Islands Corporation.

        "HUTCHINSON USA INVESTOR AGREEMENT" means the Investor Agreement dated
June 23, 1999 among Hutchinson USA, Hutchinson Telecommunications Limited and
the Company.

        "PERMITTED TRANSFEREE" means any entity in which Investor owns, directly
or indirectly, more than 40% of the outstanding voting power and of which
members of the Investor Group collectively are the largest shareholder.

        "PERSON" means an individual, partnership, joint-stock company,
corporation, trust or unincorporated organization, limited liability company, or
a government or agency or political subdivision thereof or any other entity.

        "QUALIFIED DESIGNEE" means an individual designated by Investor,
provided that the Company shall have the right to approve such designee, which
approval shall not be unreasonably withheld, so long as such individual's
membership on the Board shall not cause any violation of any Federal anti-trust
law or any other Federal or state law.

        "REORGANIZATION" has the meaning given in the Omnipoint Reorganization
Agreement.

        "SALE" means (i) a public offer and sale or other public distribution of
securities, including pursuant to an effective registration statement under the
Securities Act, or (ii) an offer or sale not involving a public offering
pursuant to an exemption from registration under Rules 144 or 145 or otherwise
under the Securities Act.




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        "STANDSTILL PERIOD" has the meaning given in Section 3(a)(i).

        "STANDSTILL TERMINATION EVENT" means the date on which the first of the
following occurs: (i) the Investor Group Beneficially Owns, in the aggregate,
less than ten percent (10%) of the Voting Power, (ii) the Investor Group
Beneficially Owns, in the aggregate, more than 90% of the Voting Power, (iii) a
Change of Control or (iv) a Bankruptcy Event.

        "SUBSIDIARY" means, as to any Person, another Person which is an entity
as to which such Person owns more than fifty percent (50%) of the outstanding
voting power and more than fifty percent (50%) of the equity.

        "THIRD PARTY CHANGE OF CONTROL" has the meaning given in Section
3(a)(i)(C).
        "THIRD PARTY OFFER" has the meaning given in Section 3(a)(i)(C).

        "THRESHOLD PERCENTAGE" means nineteen and nine tenths percent (19.9%);
provided that the Threshold Percentage shall be less to the extent necessary if
such ownership would cause a violation of the spectrum cap limits set by 47
C.F.R. Section 20.6 or other legislation or rule or that will require
divestiture by VoiceStream or the Company (including for this purpose the Cook
Inlet Parties) pursuant to said rule, as such rule may be modified or replaced
from time to time; provided that, in such event, upon the request of Investor,
VoiceStream and the Company shall use commercially reasonable efforts and
cooperate with the Investor Group to seek to obtain appropriate waivers from
such FCC rules or effect other commercially reasonable arrangements which would
permit the Investor Group to increase its ownership percentage to a maximum of
nineteen and nine tenths percent (19.9%). The Threshold Percentage shall be
calculated by dividing the Voting Power of the Voting Securities which are
Beneficially Owned by the Investor Group by the Voting Power as of the date of
determination.

        "TRANSFER" means any sale, assignment, pledge, hypothecation, or other
transfer, disposition or encumbrance of any interest (and includes an exchange
of shares in a merger, consolidation or similar transaction).

        "TRIGGERING PERSON" has the meaning given in Section 3(b)(i)(A).

        "VOTING AGREEMENT" means any voting or similar agreement to which any
member of the Investor Group and either or both of the Company or VoiceStream
are party from time to time which provides, among other things, for the voting
of securities for the election of directors of VoiceStream or the Company.

        "VOTING POWER" means, as of the date of determination, the total number
of votes which may be cast in the election of directors of the Company at any
meeting of shareholders of the Company if held on such date of determination if
all Voting Securities then outstanding were present and voted to the fullest
extent possible at such meeting. Voting Power shall be determined based on
information included in the Company's or VoiceStream's most recently filed Form
10-K or Form 10-Q notwithstanding subsequent changes thereto unless such changes
have been reported on a Form 8-K or reported to Investor in writing by the
Company or VoiceStream.




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        "VOICESTREAM" means VoiceStream Wireless Corporation, a Washington
corporation.

        "VOTING SECURITY" means, as of the date of determination, the Common
Stock of the Company, any other security generally entitled to vote for the
election of directors.

        "13D GROUP" means any group of persons formed for the purpose of
acquiring, holding, voting or disposing of Voting Securities which would be
required under Section 13(d) of the Exchange Act, and the rules and regulations
promulgated thereunder, to file a statement on Schedule 13D or a Schedule 13G
with the Commission as a "person" within the meaning of Section 13(d)(3) of the
Exchange Act if such group Beneficially Owned (as defined without excluding
paragraph (d) of Rule 13d-3) sufficient securities to require such a filing
under the Exchange Act. When references herein are to a group under Section
13(d) and not to members of such group, such references shall be deemed to refer
to actions of the group acting as such group and not to the individual actions
of any members of such group, unless and to the extent such actions would not be
permitted to be taken by the group.

        When a reference is made in this Agreement to a Section, such reference
shall be to a Section of this Agreement unless otherwise indicated. Whenever the
words "include," "includes" or "including" are used in this Agreement, they
shall be deemed to be followed by the words "without limitation." The use of a
gender herein shall be deemed to include the neuter, masculine and feminine
genders whenever necessary or appropriate. Whenever the word "herein,"
"hereunder" or "hereof" is used in this Agreement, it shall be deemed to refer
to this Agreement and not to a particular Section of this Agreement unless
expressly stated otherwise.

3.      STANDSTILL.

        (a)    STANDSTILL OBLIGATIONS.

               (i) LIMITATION. Unless there shall have occurred a Standstill
Termination Event, until the fifth (5th) anniversary of the date of this
Agreement (the "STANDSTILL PERIOD"), except with Disinterested Board Approval,
no member of the Investor Group shall, directly or indirectly,

               (A) acquire or agree to acquire any Voting Securities (except by
        way of stock splits, stock dividends or other distributions) if, in any
        such case, the effect of such acquisition would be to increase the
        Investor's Ownership Percentage to more than the Threshold Percentage;

               (B) solicit proxies with respect to the Voting Securities or
        become a "PARTICIPANT" in any "ELECTION CONTEST" (as such terms are used
        in Rule 14(a)-11 of Regulation 14A promulgated under the Exchange Act)
        relating to the election of directors of the Company, it being
        understood that the Investor Group shall not be deemed to be such a
        participant merely by reason of the membership of any Investor Directors
        on the Board pursuant to the terms of any Voting Agreement; provided,
        that if VoiceStream or the Company shall be in breach under any Voting
        Agreement (which breach, in thereasonable judgment of Investor, could
        likely result in an Investor Director not being elected in




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        accordance with the terms of the Voting Agreement), the Investor Group
        may engage in such activities for the limited purpose of electing the
        Investor Directors; or

               (C) join a 13D Group (other than a 13D Group which includes all
        of the Current Principal Stockholders) with any Person which is not a
        member of the Investor Group or otherwise induce, attempt to induce or
        in any manner act in concert with any such Person for the purpose of
        initiating or effectuating a tender offer or exchange offer for any
        Voting Securities (a "THIRD PARTY OFFER") or a transaction which would
        result in a Change of Control (a "THIRD PARTY CHANGE OF CONTROL");
        provided that the provisions of this clause (C) shall not be applicable
        if any Current Principal Stockholder holding more than 5% of the Voting
        Power shall have engaged in any material respect in any of the
        activities referred to in this clause (C).

               (D) either alone or through or with any Person initiate, induce,
        attempt to induce or in any manner act in concert with or otherwise
        support, encourage or act in concert with, any such Person for the
        purpose of initiating or effectuating a tender or exchange offer or
        Change of Control; or

               (E) disclose to any Person any intention, plan or arrangement
        inconsistent with the foregoing.

        Nothing in this Section 3(a)(i) shall have the effect of (i) precluding
a member of the Investor Group from participating in a Third Party Offer or
voting or agreeing to vote its shares in favor of a Third Party Change of
Control in which the Investor Group would receive consideration on the same
basis as is generally available to other holders of Common Stock or (ii)
prohibiting any Investor Director (acting in such capacity) from participating
(A) in discussions with other members of the Board or (B) in meetings of the
Board.

        If any member of the Investor Group makes such an acquisition or
exercise that would increase the percentage interest of the Investor Group in
the Voting Power to more than the Threshold Percentage, such excess shares shall
(for so long as the Voting Power exceeds the Threshold Percentage) be voted in a
manner proportionate to shares voted by the shareholders of the Company other
than the Investor Group and the Permitted Transferees; provided, that if the
excess shares shall result from the bad faith actions of the Investor Group or
its Affiliates, it shall promptly divest such excess; provided, further,
however, that no member of the Investor Group shall be obligated to divest
itself of such excess pursuant to this Section 3(a)(i) until such time as such
divestment would not subject such member of the Investment Group to liability
under Section 16(b) of the Exchange Act or any other applicable provision of
Federal or state law.

               (ii) RECAPITALIZATIONS, ETC. Notwithstanding Section 3(a)(i), no
member of the Investor Group shall be obligated to dispose of any Voting
Securities if the aggregate percentage ownership of the Investor Group is
increased as a result of (x) a recapitalization, merger, consolidation or other
reorganization of the Company, (y) a repurchase of Voting Securities by the
Company or (z) any other action taken by the Company or its Affiliates other
than the Investor Group.




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               (iii) REINSTATEMENT OF STANDSTILL. If a Standstill Termination
Event shall have occurred by virtue of the Investor Group Beneficially Owning
less than ten percent (10%) of the Voting Power and thereafter the Investor
Group shall Beneficially Own more than ten percent (10%) of the Voting Power
(other than as a result of (x) a recapitalization, merger, consolidation or
other reorganization of the Company, (y) a repurchase of Voting Securities by
the Company or (z) any other action taken by the Company or its Affiliates other
than the Investor Group), the provisions of this Section 3 shall be deemed to
have been reinstated.

        (b)     EXCEPTION FOR CERTAIN THIRD-PARTY ACQUISITIONS.

               (i) EXCEPTION TO STANDSTILL OBLIGATION. Notwithstanding Section
3(a)(i), the Investor Group may:

               (A) acquire Voting Securities without regard to the limitations
        set forth above but in accordance with Section 3(b)(ii) if at any time
        any person or 13D Group of persons, (other than any 13D Group which
        includes Investor or any of its Affiliates) (such person or persons
        together with any of their Affiliates, collectively, a "TRIGGERING
        PERSON"), directly or indirectly, (x) makes a bona fide offer to
        acquire, or (y) acquires, Beneficial Ownership of Voting Securities
        which, if added to the Voting Securities (if any) already Beneficially
        Owned by such Triggering Person, would represent ownership of Voting
        Securities greater than the Threshold Percentage or, in the event the
        Triggering Person is Hutchinson USA, greater than the "Threshold
        Percentage" specified in the Hutchinson USA Investor Agreement, which is
        applicable at such time;

                (B) with Disinterested Board Approval, make an Investor Tender
        Offer during the Standstill Period; and

               (C) with Disinterested Board Approval, acquire Voting Securities
        (including stock options, warrants or rights to purchase Voting
        Securities) without regard to the limitations set forth above.

The Company shall give Investor written notice of the occurrence of any event of
the type referred to in clause (A) promptly after it obtains knowledge of such
event.

               (ii) COMPETING OFFERS. If an event identified in Section
3(b)(i)(A) occurs and shall not have been withdrawn or terminated, the Investor
Group shall be permitted to take such action and make such offers as may be
considered to be of the same nature and type of action or offer and for the same
resulting number of shares as that which is being taken by the Triggering
Person; provided that the Investor Group may only acquire that number of shares
which when added to the number of shares already owned by the Investor Group
shall not exceed the number of shares Beneficially Owned (as defined without
excluding paragraph (d) of Rule 13d-3) and to be acquired (assuming any
proposals or offers to purchase have been consummated) by the Triggering Person.
In proceeding with any action or offer permitted under this Section 3(b)(ii),
the Investor Group shall be permitted to offer more favorable terms such as
price, cash versus securities or other such terms as may be consistent with an
offer of the same nature and type of consideration as that which is being
proposed by the Triggering Person.




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               (iii) NO CONTESTING. If the Investor Group shall take any such
action permitted by this Section 3(b), the Company agrees that it shall not in
any way (whether by active opposition, Board announcement or otherwise) contest
such action, subject in all events to the fiduciary obligations of the Company's
Board and officers to the Company's stockholders.

        (c)    OPTION TO PURCHASE INCREMENTAL SHARES.

               (i) INCREMENTAL SHARES. If an event identified in Section 3(b)(i)
occurs and as a result the Investor Group acquires Voting Securities which
increase the Investor Group's percentage interest in the Voting Power to more
than the Threshold Percentage (the "INCREMENTAL SHARES"), and thereafter the
Triggering Person holds Voting Securities representing a percentage of the
Voting Power less than the Threshold Percentage, then, upon the expiration of
the Investor Group's right to dispose of the Incremental Shares as provided in
Section 3(c)(ii) below, the Investor Group hereby grants to the Company or a
designee selected with Disinterested Board Approval, for a period of ninety (90)
days (subject to extension in the event of Investor's exercise of rights under
Section 3(c)(ii) below), an option to acquire any Incremental Shares at a price
equal to the price paid by the Investor Group for such shares, plus such
expenses and costs reasonably necessary to acquire the Incremental Shares and
incurred by the Investor Group in acquiring the Incremental Shares (including
the Imputed Cost of Funds of the Investor Group of holding the Incremental
Shares until acquired by the Company or such designee); provided, however, that
the Investor Group shall not be obligated to sell any Voting Securities pursuant
to this Section 3(c)(i) until such time as such sale would not subject the
Investor Group to liability under Section 16(b) of the Exchange Act or any other
applicable provision of Federal or state law.

               (ii) DISPOSITION. In the event that the Investor Group has
acquired Incremental Shares from a seller or sellers other than the Company, for
a period of sixty (60) days from the date on which the Investor Group holds
Incremental Shares, the Investor Group shall have the right to sell such
Incremental Shares as follows: (x) to an independent third party in a bona fide
transaction or transactions; (y) if Rule 144 is available, into the public
market in accordance with the terms of Rule 144; or (z) as provided under both
(x) and (y). In the event that the Investor Group elects to dispose of the
Incremental Shares as provided in this Section 3(c)(ii), Investor shall provide
written notice to the Company of such disposition and the purchase option
granted to the Company pursuant to Section 3(c)(i) shall apply only to those
Incremental Shares which have not been so disposed of; provided, however, that
the Investor Group shall not be obligated to sell any Voting Securities pursuant
to this Section 3(c)(ii) until such time as such sale would not subject the
Investor Group to liability under Section 16(b) of the Exchange Act or any other
applicable provision of Federal or state law.

               (iii) ADJUSTMENT FOR FAILURE TO EXERCISE OPTION. In the event
that the Company or its designee fails to exercise its option as provided in
Section 3(c)(i), the Threshold Percentage shall be increased to a percentage
equal to the percentage of the Voting Power held by the Investor Group upon the
expiration of the right to exercise such option by the Company or such designee.




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               (iv) VOTING OF INCREMENTAL SHARES. Until the Incremental Shares
shall have been disposed of as provided in Section 3(c)(ii) and, if applicable,
the option of the Company or its designee provided in Section 3(c)(i) shall have
expired, the Incremental Shares shall be voted in a manner proportionate to
shares voted by the shareholders of the Company other than the Investor Group
and the Permitted Transferees.

               (v) NO CIRCUMVENTION. (a) The Investor Group shall not attempt to
circumvent the provisions of this Section 3(c) by taking any action that would
have the effect of extending the periods for which Section 16(b) liability would
apply.

               (b) The Investor Group shall not acquire or agree to acquire any
Voting Securities of, or other equity interest in, the Cook Inlet Parties except
as a member of a 13D Group which includes all of the Current Principal
Stockholders..

4.      TRANSFER.

        (a) The Voting Securities Beneficially Owned by the Investor Group shall
be freely tradable and may be Transferred by the Investor Group in Sales as
provided herein or in the Registration Rights Agreement, provided that the
Investor Group takes reasonable care in approving a plan of distribution for
sales of Common Stock pursuant to a registration statement or authorizing sales
pursuant to Rules 144 or 145 or otherwise to preclude the acquisition of more
than five percent (5%) of the Voting Power by any Person, Affiliate or 13D Group
(other than the Current Principal Stockholders and their Permitted Affiliate
Transferees (as defined in the Voting Agreement) and, with respect to Hutchinson
USA, the Disqualified Affiliates (as defined in the Hutchinson USA Investor
Agreement) unless otherwise approved by Disinterested Board Approval. Any Sale
not made pursuant to a registration statement as permitted by the Registration
Rights Agreement shall be made in compliance with Rule 144 and/or 145 or
pursuant to another exemption from registration.

        (b) Provided that it shall have provided prior written notice to the
Company, the Investor Group and the Permitted Transferees shall have the
unrestricted right to Transfer their Voting Securities and the rights and
obligations hereunder to members of the Investor Group and the Permitted
Transferees (provided that any such transferees agree in writing to be bound by
the terms of this Agreement).

        (c) Nothing herein shall restrict in any way the Transfer of any
derivative security with respect to the Voting Securities Beneficially Owned by
the Investor Group and the Permitted Transferees.

5.      COVENANTS OF THE INVESTOR.

        Until the termination of the Aerial Reorganization Agreement in
accordance with the terms thereof, Investor agrees as follows:

               (a) At any stockholders meeting of Aerial (or at any adjournment
        thereof) or in any other circumstances upon which a vote, consent or
        other approval with




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<PAGE>   12

        respect to the Aerial Reorganization or the Aerial Reorganization
        Agreement is sought, the Investor shall vote (or cause to be voted) the
        shares of Aerial Common Stock owned by it or its Affiliates in favor of
        the Aerial Reorganization, the approval and adoption of the Aerial
        Reorganization Agreement and the approval of the terms thereof and each
        of the other transactions contemplated by the Aerial Reorganization
        Agreement.

               (b) At any meeting of stockholders of Aerial (or at any
        adjournment thereof) or in any other circumstances upon which a vote,
        consent or other approval is sought, other than with respect to the
        Aerial Reorganization or Aerial Reorganization Agreement, the Investor
        shall vote (or cause to be voted) the shares of Aerial Common Stock
        owned by it or its Affiliates against any merger agreement or merger,
        consolidation, sale of all or substantially all of the assets of Aerial,
        or reorganization, recapitalization, dissolution, liquidation or winding
        up of or by Aerial or any Subsidiary of Aerial or any other Acquisition
        Proposal. The Investor further agrees not to commit or agree to take any
        action inconsistent with the foregoing.

               (c) The Investor agrees not to (i) sell transfer, pledge,
        encumber, assign or otherwise dispose of (including by gift)
        (collectively, "Transfer"), or enter into any contract, option or other
        arrangement (including any profit-sharing arrangement) with respect to
        any Transfer of the shares of Aerial Common Stock owned by it or its
        Affiliates to any person or (ii) enter into any voting arrangement,
        whether by proxy, voting agreement or otherwise (collectively, "Voting
        Agreement"), in relation to the shares of Aerial Common Stock owned by
        it or its Affiliates, and agrees not to commit or agree to take any of
        the foregoing actions. Notwithstanding the foregoing, the Investor may
        make Transfers to Permitted Transferees.

               (d) The Investor shall not, nor shall the Investor authorize any
        affiliate, director, officer, employee, investment banker, attorney or
        other advisor or representative of the Investor to, (i) directly or
        indirectly solicit, initiate or encourage the submission of, any
        Acquisition Proposal or, (ii) directly or indirectly participate in any
        discussions or negotiations regarding, or furnish to any person any
        information with respect to, or take any other action to facilitate any
        inquiries or the making of any proposal that constitutes or may
        reasonably be expected to lead to, any Acquisition Proposal or (iii)
        directly or indirectly take or participate in any actions set forth in
        Section 5.3 of the Aerial Reorganization Agreement.

               (e) The Investor shall use all reasonable efforts to take, or
        cause to be taken, all actions, and to do, or cause to be done, and to
        assist and cooperate in doing, all things necessary, proper or advisable
        to consummate and make effective, in the most expeditious manner
        practicable, the Aerial Reorganization and the other transactions
        contemplated by the Aerial Reorganization Agreement.

               (f) Investor hereby irrevocable waives any rights of appraisal
        or rights to dissent from the Aerial Reorganization that Investor may
        have.




                                      -11-
<PAGE>   13

               (g) Sonera agrees to execute at the closing of the Omnipoint
        Reorganization and again at the closing of the Aerial Reorganization a
        tax certificate in the form attached HERETO.

6.      VOTING AGREEMENT AND DIRECTOR DESIGNEES.

               (a) Certain Stockholders of VoiceStream ("Parent Stockholders")
        are parties to a Voting Agreement, dated May 3, 1999 ("VoiceStream
        Voting Agreement"), pursuant to which each Parent Stockholder agreed on
        the terms set forth in the VoiceStream Voting Agreement to vote the
        shares of VoiceStream Common Stock Beneficially Owned by it at the time
        of a vote in favor of directors designated by such Parent Stockholders.
        On June 23, 1999 the Parent Stockholders entered into an Agreement (the
        "Omnipoint Voting Agreement") with certain stockholders of Omnipoint
        (the "Omnipoint Stockholders") in which they agreed, among other things,
        to terminate the VoiceStream Voting Agreement and enter into a new
        Voting Agreement on terms mutually satisfactory to Omnipoint
        Stockholders and Parent Stockholders ("Newco Voting Agreement") which
        will set forth voting arrangements which will apply to the Company after
        the Omnipoint Reorganization. The Parent Stockholders and Investor
        hereby agree to enter into a voting agreement ("Newco Voting Agreement
        II") effective on the Effective Time of the Omnipoint Reorganization on
        terms mutually satisfactory to the Parent Stockholders and Investor,
        pursuant to which (w) the voting arrangements which existed under the
        VoiceStream Voting Agreement will apply to the Company, (x) the
        provisions of Section 6 (b) below shall also be effectuated, (y) the
        provisions of the letter agreement, dated June 23, 1999 ("Hutchinson
        Letter"), with Hutchinson will be effectuated, and (z) upon consummation
        of the Omnipoint Reorganization, the provisions of Section 7.4 of the
        Omnipoint Agreement shall be effectuated. On September 17, 1999, Parent
        Stockholders, Aerial, TDS, VoiceStream and the Company entered into a
        Parent Stockholders Agreement, whereby, among other things, Parent
        Stockholders and TDS agreed, effective on the Effective Time of the
        Aerial Reorganization, to enter into a voting agreement substantially
        similar to that contemplated hereby pursuant to which, in addition to
        the provisions referred in clauses (w), (y) and (z) above, the Principal
        Stockholders and TDS agreed to vote for one director designated by TDS.
        The Parent Stockholders and Investor shall use reasonable efforts to
        seek to have the Omnipoint Stockholders enter into Newco Voting
        Agreement II, effective on the Effective Time of the Omnipoint
        Reorganization, on terms mutually satisfactory to the Parent
        Stockholders, Investor and the Omnipoint Stockholders. If the Omnipoint
        Stockholders do not enter into Newco Voting Agreement II effective at
        the Effective Time of the Omnipoint Reorganization, the Parent
        Stockholder and Investor shall enter into Newco Voting Agreement II
        effective at the Effective Time of the Omnipoint Reorganization, it
        being understood and agreed that the Parent Stockholders and the
        Omnipoint Stockholders will still enter into the Newco Voting Agreement.
        The Parent Stockholders and Investor shall use reasonable efforts to
        seek to have TDS enter into Newco Voting Agreement II, effective on the
        Effective Time of the Aerial Reorganization, on terms mutually
        satisfactory to the Parent Stockholders, Investor and TDS. If TDS does
        not enter into Newco Voting Agreement II effective at the Effective Time
        of the Aerial Reorganization, the Parent Stockholder and Investor shall
        enter into Newco Voting Agreement II effective at the Effective Time of
        the Omnipoint




                                      -12-
<PAGE>   14

        Reorganization, it being understood and agreed that the Parent
        Stockholders and TDS will still enter into an appropriate voting
        agreement

               (b) Pursuant to Newco Voting Agreement II each of the Parent
        Stockholders and Investor (and the Omnipoint Stockholders if they agree
        to enter into such agreement) shall agree, on the terms set forth
        therein, to vote, or cause to be voted, all of the shares of Parent
        Common Stock Beneficially Owned by it at the time of the vote in person
        or by proxy (and shall take all other necessary or desirable action
        within the Investor's or such Parent Stockholder's control including
        attendance at meetings in person or by proxy for purposes of obtaining a
        quorum and execution of written consents in lieu of meetings), for the
        election and continuation in office of (i) one (1) Qualified Designee as
        a director of the Company so long as the Investor Beneficially Owns at
        least 4,500,000 shares of Parent Common Stock; provided, however, if
        Investor owns more than 9,800,000 shares of Parent Common Stock and TDS
        owns less than 4,500,000 shares of Parent Common Stock, Investor shall
        be permitted to designate two (2) Qualified Designees; and (ii) the
        directors designated by the Parent Stockholders pursuant to the
        VoiceStream Voting Agreement (as restated in Newco Voting Agreement II),
        the Hutchinson Letter and Section 7.4 of the Omnipoint Agreement.

               (c) Parent agrees if necessary, to amend the Bylaws of Parent,
        to increase the number of authorized directors to a number sufficient to
        satisfy the obligations in the VoiceStream Voting Agreement, Newco
        Voting Agreement and Newco Voting Agreement II, as applicable.

7.      MISCELLANEOUS.

        (a) WAIVER; AMENDMENTS. Except as expressly provided otherwise herein,
neither this Agreement nor any provision hereof may be changed, waived,
discharged or terminated orally, but only by an instrument in writing signed by
the Company and the Investor.

        (b) RECAPITALIZATION, EXCHANGES, ETC. The provisions of this Agreement
shall apply to the full extent set forth herein with respect to shares or other
securities of the Company that may be issued to the Investor in respect of, in
exchange for, or in substitution of the Common Stock.

        (c) SPECIFIC PERFORMANCE. Each of the parties hereto acknowledges and
agrees that, in the event of any breach of this Agreement, the non-breaching
parties would be irreparably harmed and could not be made whole by monetary
damages. Accordingly, each party hereto agrees that the other party, in addition
to any other remedy to which they may be entitled at law or in equity, shall be
entitled to compel specific performance of this Agreement.

        (d) NOTICES. All notices, requests, demands and other communications
hereunder shall be in writing and, except to the extent otherwise expressly
provided in this Agreement, shall be deemed to have been duly given if delivered
by same day or next day courier (guaranteed delivery) or mailed, registered
mail, return receipt requested, or transmitted by telegram, telex or facsimile
(i) if to the Investor, at the Investor's address appearing below or at any
other address




                                      -13-
<PAGE>   15

the Investor may have provided in writing to the Company and (ii) if to the
Company, at 3650 131st Avenue S.E., Suite 400, Bellevue, WA 98006, U.S.A., Tel:
(425) 586-8014, Fax: (425) 586-8080; Attention: General Counsel, or such other
address as the Company may have furnished to the Investor in writing, with
copies (which shall not constitute notice) to Preston Gates & Ellis LLP, 5000
Columbia Center, 701 Fifth Avenue, Seattle, WA 98104, Attention: Richard B.
Dodd, Esq., Facsimile No.: (206) 623-7022 and to Friedman Kaplan & Seiler LLP,
875 Third Avenue New York, NY 10022 Attention: Barry A. Adelman, Esq. Facsimile
No.: (212) 355-6401. If a notice hereunder is transmitted by confirmed fax so as
to arrive during normal business hours during a Business Day at the place of
receipt, then such notice shall be deemed to have been given on such Business
Day at the place of receipt or, if so transmitted to arrive after normal
business hours during a Business Day at the place of receipt, then such notice
shall be deemed to have been given on the following Business Day at the place of
receipt. If such notice is sent by next-day courier it shall be deemed to have
been given on the next Business Day at the place of receipt following sending
and, if by registered mail, on the fifth Business Day at the place of receipt
following sending, provided, that the date of sending shall be deemed to be the
date at the place of receipt at the time such notice is posted.

The Investor Group:

                             Sonera Ltd.
                             P.O. Box, Fin - 00051 - Tele
                             Teollisuuskatu 15
                             Helsinki, Finland
                             Attention:  Maire Laitinen, Esq.
                             Facsimile No.: 011-358-2040-3414

                      with a copy to:

                             Patton Boggs LLP
                             2550 M. St. N.W.
                             Washington, D.C.  20037

                             Attention: Richard M. Stolbach, Esq.
                             Facsimile No.:  (202) 457-6315

        (f) SUCCESSORS AND ASSIGNS. Except as otherwise provided herein, this
Agreement shall inure to the benefit of, and be binding upon, the successors and
assigns of each of the parties; provided, however, that this Agreement may not
be assigned by any party hereto other than in compliance with the terms hereof.

        (g) COUNTERPARTS. This Agreement may be executed in one or more
counterparts, each of which shall be deemed an original and all of which
together shall be considered one and the same agreement.

        (h) ENTIRE AGREEMENT. This Agreement constitutes the entire
understanding of the parties hereto with respect to the subject matter hereof
and supersedes all prior understandings among such parties with respect to such
subject matter.




                                      -14-
<PAGE>   16

        (i) APPLICABLE LAW. The validity of this Agreement, its construction,
interpretation and enforcement, and the rights of the parties hereunder, shall
be determined under, governed by and construed in accordance with the internal
laws of the State of Delaware applicable to contracts formed in such State. Each
party hereto agrees that any suit, action or other proceeding arising out of
this Agreement shall be brought and litigated in the courts of the State of
Delaware or the United States District Court for the District of Delaware and
each party hereto hereby irrevocably consents to exclusive personal jurisdiction
and venue in any such court and hereby waives any claim it may have that such
court is an inconvenient forum for the purposes of any such suit, action or
other proceeding.

        (j) 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.

        (k) FAILURE TO PURSUE REMEDIES. The failure of any party to seek redress
for violation of, or to insist upon the strict performance of, any provision of
this Agreement shall not prevent a subsequent act, which would have originally
constituted a violation, from having the effect of an original violation.

        (l) 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.

        (m) 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 provision were omitted.

            [The remainder of this page was intentionally left blank]




                                      -15-
<PAGE>   17

        IN WITNESS WHEREOF, the parties hereto have executed this Investor
Agreement as of the date first above written.

                             SONERA LTD.

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

                             VOICESTREAM WIRELESS CORPORATION

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

                             VOICESTREAM WIRELESS HOLDING

                             CORPORATION

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




                                      -16-
<PAGE>   18

                                   SCHEDULE I

                               PARENT STOCKHOLDERS

NAME AND ADDRESS OF STOCKHOLDER
-------------------------------

Hellman & Friedman Capital Partners II, L.P.
c/o Hellman & Friedman LLC
One Maritime Plaza, Suite 1200
San Francisco, California 94111
Attention: Mitchell R. Cohen
Fax:  415-788-0176

H&F Orchard Partners, L.P.
c/o Hellman & Friedman
One Maritime Plaza, Suite 1200
San Francisco, California 94111
Attention: Mitchell R. Cohen
Fax:  415-788-0176

H&F International Partners, L.P.
c/o Hellman & Friedman
One Maritime Plaza, Suite 1200
San Francisco, California 94111
Attention: Mitchell R. Cohen
Fax:  415-788-0176

GS Capital Partners, L.P.
c/o Goldman, Sachs & Co.
85 Broad Street
New York, NY 10004
Attention:  Terence O'Toole
Fax: 212-902-3000

The Goldman Sachs Group, Inc.
c/o Goldman, Sachs & Co.
85 Broad Street
New York, NY 10004
Attention:  Terence O'Toole
Fax: 212-902-3000




                                      -17-
<PAGE>   19
NAME AND ADDRESS OF STOCKHOLDER
-------------------------------

Bridge Street Fund 1992, L.P.
c/o Goldman, Sachs & Co.
85 Broad Street
New York, NY 10004
Attention:  Terence O'Toole
Fax: 212-902-3000

Stone Street Fund 1992, L.P.
c/o Goldman, Sachs & Co.
85 Broad Street
New York, NY 10004
Attention:  Terence O'Toole
Fax: 212-902-3000

Providence Media Partners L.P.
c/o Providence Ventures, Inc.
900 Fleet Center
50 Kennedy Plaza
Providence, Rhode Island 02903
Attention: Jonathan Nelson
Fax: 401-751-1790

John W. Stanton and Theresa E. Gillespie
c/o VoiceStream Wireless Corporation
3650 131st Avenue S.E., Suite 400
Bellevue, WA 98006
Attention:  John W. Stanton
Fax: 425-586-8010

PN Cellular, Inc.
c/o VoiceStream Wireless Corporation
3650 131st Avenue S.E., Suite 400
Bellevue, WA 98006
Attention:  John W. Stanton
Fax: 425-586-8010

Stanton Family Trust
c/o VoiceStream Wireless Corporation
3650 131st Avenue S.E., Suite 400
Bellevue, WA 98006
Attention:  John W. Stanton
Fax: 425-586-8010




                                      -18-
<PAGE>   20
Stanton Communications Corporation
c/o VoiceStream Wireless Corporation
3650 131st Avenue S.E., Suite 400
Bellevue, WA 98006
Attention:  John W. Stanton
Fax: 425-586-8010

Hutchinson Telecommunications
PCS (USA) Limited
c/o Offshore Incorporations Limited
P.O. Box 957
Offshore Incorporations Centre
Road Town, Tortola
British Virgin Islands
Telephone No.: 809-494-2233
Facsimile No.: 809-494-4885

Hutchinson Telecommunications Holdings (USA) Limited
c/o Offshore Incorporations Limited
P.O. Box 957
Offshore Incorporations Centre
Road Town, Tortola
British Virgin Islands
Telephone No.: 809-494-2233
Facsimile No.: 809-494-4885




                                      -19-