<PAGE>   1

                                                                     Exhibit 4.3








                                WARRANT AGREEMENT


                         dated as of [________ __, ____]


                                     between


                             XO COMMUNICATIONS, INC.


                                       and


                    [NAME OF WARRANT AGENT], as Warrant Agent



                              Warrants to Purchase
                          [TITLE OF WARRANT SECURITIES]


<PAGE>   2


                                TABLE OF CONTENTS

<TABLE>
<S>                                                                                                                        <C>
ARTICLE I       ISSUANCE OF WARRANTS AND FORM, EXECUTION, DELIVERY AND REGISTRATION OF WARRANT CERTIFICATES..................1

        Section 1.1     Issuance of Warrants.................................................................................2

        Section 1.2     Form, Execution and Delivery of Warrant Certificates.................................................2

        Section 1.3     Transfer of Warrants.................................................................................3

        Section 1.4     Lost, Stolen, Mutilated or Destroyed Warrant Certificates............................................5

        Section 1.5     Cancellation of Warrant Certificates.................................................................5

        Section 1.6     Treatment of Holders.................................................................................6

ARTICLE II      EXERCISE PRICE, DURATION AND EXERCISE OF WARRANTS............................................................6

        Section 2.1     Exercise Price.......................................................................................6

        Section 2.2     Duration of Warrants.................................................................................6

        Section 2.3     Exercise of Warrants.................................................................................7

        Section 2.4     Cancellation.........................................................................................9

ARTICLE III     OTHER PROVISIONS RELATING TO RIGHTS OF HOLDERS...............................................................9

ARTICLE III [If Warrants are to be issued in Book-Entry Form: AND BENEFICIAL OWNERS] OF WARRANTS.............................9

        Section 3.1     No Rights as Holders of Warrant Securities Conferred by Warrants or Warrant Certificates.............9

        Section 3.2     Holder [If Warrants are to be issued in Book-Entry form: and Beneficial Owner] of Warrant
                        May Enforce Rights..................................................................................10

ARTICLE IV      CONCERNING THE WARRANT AGENT................................................................................10

        Section 4.1     Warrant Agent.......................................................................................10

        Section 4.2     Limitations on Warrant Agent's Obligations..........................................................10

        Section 4.3     Compliance With Applicable Laws.....................................................................12

        Section 4.4     Resignation and Appointment of Successor............................................................12

ARTICLE V       MISCELLANEOUS...............................................................................................13

        Section 5.1     Amendments..........................................................................................13

        Section 5.2     Merger, Consolidation, Sale, Transfer or Conveyance.................................................14

        Section 5.3     Notices and Demands to the Company and Warrant Agent................................................15

        Section 5.4     Addresses...........................................................................................15

        Section 5.5     Governing Law.......................................................................................15
</TABLE>


                                       -i-
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                                TABLE OF CONTENTS
                                  (continued)


<TABLE>
<CAPTION>
                                                                                                                          PAGE
        <S>             <C>                                                                                                <C>
        Section 5.6     Delivery of Prospectus..............................................................................15

        Section 5.7     Obtaining of Governmental Approvals.................................................................15

        Section 5.8     Payment of Taxes....................................................................................15

        Section 5.9     Benefits of Warrant Agreement.......................................................................16

        Section 5.10    Headings............................................................................................16

        Section 5.11    Severability........................................................................................16

        Section 5.12    Counterparts........................................................................................16

        Section 5.13    Inspection of Agreement.............................................................................16
</TABLE>








                                      -ii-
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                                WARRANT AGREEMENT

                WARRANT AGREEMENT, dated as of [________ __, ____] (as modified,
amended or supplemented, this "Agreement"), between XO COMMUNICATIONS, INC., a
Delaware corporation (the "Company") and [NAME OF WARRANT AGENT], a
[____________], as Warrant Agent (the "Warrant Agent").

                              W I T N E S S E T H:

                [If offer consists of Debt Securities with Warrants AND/OR
Warrants to Purchase Debt Securities: WHEREAS, the Company has entered into an
Indenture (the "[Senior] [Subordinated] Indenture"), dated as of [____________],
between the Company and [NAME OF TRUSTEE] as trustee (the "[Senior]
[Subordinated] Trustee"), providing for the issuance from time to time of its
unsecured [senior] [subordinated] [convertible] debentures, notes or other
evidences of indebtedness (the "[Senior] [Subordinated] Debt Securities"), to be
issued in one or more series as provided in the [Senior] [Subordinated]
Indenture [if Warrant Securities are not under same Indenture as Debt Securities
to which they are attached; and an Indenture (the "[Senior] [Subordinated]
Indenture," the Senior and Subordinated Indentures being referred to
collectively as the "Indentures") dated as of _______________ between the
Company and ______________, as trustee (the "[Senior] [Subordinated] Trustee,"
(the Senior and Subordinated Trustees being referred to collectively, as the
"Trustees"), providing for the issuance from time to time of its [senior]
[subordinated] debentures, notes or other evidences of indebtedness (the
"[Senior] [Subordinated] Debt Securities," the Senior and Subordinated Debt
Securities being referred to collectively as the "Debt Securities"), to be
issued in one or more series as provided in the [Senior] [Subordinated]
Indenture]; and]

                [If Securities and Warrants are to be offered together: WHEREAS,
the Company proposes to sell [title of Securities being offered] (the "Offered
Securities") together with warrants (each, a "Warrant") representing the right
to purchase [title of Securities purchasable upon exercise of Warrants] [If
Warrants for Depositary Shares are to be offered: , each representing a [1/__th]
interest in a share of [title of securities represented by Depositary Shares]]
(the "Warrant Securities" [If Warrants for Depositary Shares are to be offered:
, which term shall also refer, as appropriate, to such [title of securities
represented by Depositary Shares]), such warrant certificates and other warrant
certificates issued pursuant to this Agreement being herein called the "Warrant
Certificates"; and]

                [If offer consists of Warrants alone: WHEREAS, the Company
proposes to sell warrant certificates evidencing one or more warrants (each, a
"Warrant") representing the right to purchase [title of Securities purchasable
upon exercise of Warrants] [If Warrants for Depositary Shares are to be offered:
, each representing a [1/__th] interest in a share of [title of securities
represented by the Depositary Shares]] (the "Warrant Securities" [If Warrants
for Depositary Shares are to be offered: , which term shall also refer, as
appropriate, to such [title of securities represented by the Depositary
Shares]), such warrant certificates and other warrant certificates issued
pursuant to this Agreement being herein called the "Warrant Certificates"; and]

                WHEREAS, the Company desires the Warrant Agent to act on behalf
of the Company, and the Warrant Agent is willing so to act, in connection with
the issuance, transfer, exchange, exercise and cancellation of the Warrants, and
the Company wishes to set forth in this Agreement, among other things, the
provisions of the Warrants, the form of the Warrant


                                       1
<PAGE>   5


Certificates evidencing the Warrants and the terms and conditions upon which the
Warrants may be issued, transferred, exchanged, exercised and canceled;

                NOW, THEREFORE, in consideration of the premises and of the
mutual agreements herein contained, the parties hereto agree as follows:

               ARTICLE I ISSUANCE OF WARRANTS AND FORM, EXECUTION,
                DELIVERY AND REGISTRATION OF WARRANT CERTIFICATES

        Section 1.1     Issuance of Warrants. Each Warrant shall represent the
right, subject to the provisions contained herein and therein, to purchase
[________] Warrant Securities [in the aggregate principal amount of $_____] at
the Exercise Price set forth in Section 2.1. [If Securities and Warrants are to
be offered together: Warrants shall be issued in units with the Offered
Securities [If Warrants are not immediately detachable: and shall not be
separately transferable [Unless Warrants are not detachable: before [________
__, ____] (the "Detachment Date")]].] [If Warrants are to be offered separately:
Warrants shall be issued as a separate security and shall be transferable from
and after the date of issuance.] [If Warrants are to be offered in Book-Entry
form: [All] [A portion] of the Warrants shall initially be represented by one or
more global certificates (each, a "Global Warrant Certificate").] [If Securities
and Warrants are to be offered together and in definitive form: Each Warrant
Certificate included in such a unit shall evidence [_______] Warrants for each
[$_____ principal amount of] [_______] Offered Securities included in such
unit.] [If Warrants are to be offered separately and in definitive form: Each
Warrant Certificate shall evidence [_______] Warrants.]

        Section 1.2     Form, Execution and Delivery of Warrant Certificates.

                (a)     One or more Warrant Certificates evidencing Warrants to
                        purchase not more than [____] [$_______ in aggregate
                        principal amount of] Warrant Securities (except as
                        provided in Sections 1.3, 1.4 and 2.3(e)) may be
                        executed by the Company and delivered to the Warrant
                        Agent upon the execution of this Warrant Agreement or
                        from time to time thereafter [, including upon the
                        Detachment Date].

                (b)     Each Warrant Certificate, whenever issued, shall be in
                        registered form substantially in the form set forth in
                        Exhibit A hereto, with such appropriate insertions,
                        omissions, substitutions and other variations as are
                        required or permitted by this Agreement. [If Warrants
                        are issued in Book-Entry form: Each Global Warrant
                        Certificate shall bear such legend or legends as may be
                        required by the Depositary in order for it to accept the
                        Warrants for its book-entry settlement system.] Each
                        Warrant Certificate shall be printed, lithographed,
                        typewritten, mimeographed or engraved on steel engraved
                        borders or otherwise reproduced in any other manner as
                        may be approved by the officers executing the same (such
                        execution to be conclusive evidence of such approval)
                        and may have such letters, numbers or other marks of
                        identification or designation and such legends or
                        endorsements printed, lithographed or engraved thereon
                        as the officers of the Company executing the same may
                        approve (such execution to be conclusive evidence of
                        such approval) and as are not inconsistent with the
                        provisions of this Agreement, or as may be required to
                        comply with any law or with any rule or regulation made
                        pursuant thereto, or with


                                       2
<PAGE>   6


                        any regulation of any stock exchange on which the
                        Warrants [If Securities and Warrants are to be offered
                        together: , the Offered Securities] or the Warrant
                        Securities may be listed, or to conform to usage. Each
                        Warrant Certificate shall be signed on behalf of the
                        Company by its Chairman of the Board, President or any
                        Senior Vice President. The signature of any such officer
                        on any Warrant Certificate may be manual or facsimile.
                        Each Warrant Certificate, when so signed on behalf of
                        the Company, shall be delivered to the Warrant Agent
                        together with an order for the countersignature and
                        delivery of such Warrants.

                (c)     The Warrant Agent shall, upon receipt of any Warrant
                        Certificate duly executed on behalf of the Company,
                        countersign such Warrant Certificate and deliver such
                        Warrant Certificate to or upon the order of the Company.
                        Each Warrant Certificate shall be dated the date of its
                        countersignature.

                (d)     No Warrant Certificate shall be entitled to any benefit
                        under this Agreement or be valid or obligatory for any
                        purpose, and no Warrant evidenced thereby may be
                        exercised, unless such Warrant Certificate has been
                        countersigned by the manual signature of the Warrant
                        Agent. Such signature by the Warrant Agent upon any
                        Warrant Certificate executed by the Company shall be
                        conclusive evidence that such Warrant Certificate has
                        been duly issued under the terms of this Agreement.

                (e)     If any officer of the Company who has signed any Warrant
                        Certificate either manually or by facsimile signature
                        shall cease to be such officer before such Warrant
                        Certificate shall have been countersigned and delivered
                        by the Warrant Agent, such Warrant Certificate
                        nevertheless may be countersigned and delivered as
                        though the person who signed such Warrant Certificate
                        had not ceased to be such officer of the Company; and
                        any Warrant Certificate may be signed on behalf of the
                        Company by such persons as, at the actual date of the
                        execution of such Warrant Certificate, shall be the
                        proper officers of the Company as specified in this
                        Section 1.2, regardless of whether at the date of the
                        execution of this Agreement any such person was such
                        officer.

                (f)     The Holders shall [If Warrants are to be issued in
                        Book-Entry form: , except as stated below with respect
                        to Warrants evidenced by a Global Warrant Certificate,]
                        be entitled to receive Warrants in physical,
                        certificated form.

                        [If Warrants are to be issued in Book-Entry form: (g) A
                        Global Warrant Certificate may be exchanged for a new
                        Global Warrant Certificate, or one or more new Global
                        Warrant Certificates may be issued, to reflect the
                        issuance by the Company of additional Warrants. To
                        effect such an exchange, the Company shall deliver to
                        the Warrant Agent one or more new Global Warrant
                        Certificates duly executed on behalf of the Company as
                        provided in Section 1.2. The Warrant Agent shall
                        authenticate each new Global Warrant Certificate as
                        provided in Section 1.2 and shall deliver each new
                        Global Warrant Certificate to the Depositary. The
                        Warrant Agent shall cancel each Global Warrant
                        Certificate delivered to it by the Depositary in
                        exchange therefor, if any.]


                                       3
<PAGE>   7


        Section 1.3     Transfer of Warrants.

                ( )     [If Warrants are to be issued in Book-Entry form: (a)
                        [All] [A portion] of the Warrants shall initially be
                        represented by one or more Global Warrant Certificates
                        deposited with [the Depositary Trust Company] (the
                        "Depositary") and registered in the name of [Cede &
                        Co.], a nominee of the Depositary. The Depositary, or
                        such other entity as is agreed to by the Depositary, may
                        hold each Global Warrant Certificate as custodian for
                        Depositary. Except as provided for in Section 1.3(b)
                        hereof, no person acquiring Warrants traded on any
                        securities exchange with book-entry settlement through
                        the Depositary shall receive or be entitled to receive
                        physical delivery of definitive Warrant Certificates
                        evidencing such Warrants. Ownership of beneficial
                        interests in the Warrants shall be shown on, and the
                        transfer of such ownership shall be effected through,
                        records maintained by (i) the Depositary or its nominee
                        for each Global Warrant Certificate, or (ii)
                        institutions that have accounts with the Depositary
                        (such institution, with respect to a Warrant in its
                        account, a "Participant").]

                ( )     [If Warrants are to be issued in Book-Entry form: (b)
                        If the Depositary subsequently ceases to make its
                        book-entry settlement system available for the Warrants,
                        the Company may instruct the Warrant Agent regarding
                        making other arrangements for book-entry settlement. In
                        the event that the receipts are not eligible for, or it
                        is no longer necessary to have the Warrants available
                        in, book-entry form, the Warrant Agent shall provide
                        written instructions to the Depositary to deliver to the
                        Warrant Agent for cancellation each Global Warrant
                        Certificate, and the Company shall instruct the Warrant
                        Agent to deliver to the Depositary definitive Warrant
                        Certificates in physical form evidencing such Warrants.
                        Such definitive Warrant Certificates shall be in the
                        form annexed hereto as Exhibit A with appropriate
                        insertions, modifications and omissions, as provided
                        above.]

                ( )     [If Securities and Warrants are to be offered
                        together: (c) [If Warrants are not immediately
                        detachable: Prior to the Detachment Date,] Warrants may
                        be transferred or exchanged only together with the
                        Offered Security to which such Warrant is attached, and
                        only for the purpose of effecting, or in conjunction
                        with, a transfer or exchange of such Offered Security.
                        Furthermore, [If Warrants are not immediately
                        detachable: on or prior to the Detachment Date,] each
                        transfer of an Offered Security on the register relating
                        to such Offered Securities shall operate also to
                        transfer the Warrants to which such Offered Security was
                        initially attached. [If Warrants are not immediately
                        detachable: From and after the Detachment Date, the
                        above provisions shall be of no further force and
                        effect.]

                ( )     A Warrant Certificate may be transferred at the option
                        of the Holder thereof upon surrender of such Warrant
                        Certificate at the corporate trust office of the Warrant
                        Agent, properly endorsed or accompanied by appropriate
                        instruments of transfer and written instructions for
                        transfer, all in form satisfactory to the Company and
                        the Warrant Agent [If Warrants


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<PAGE>   8


                        are to be issued in Book-Entry form: ; provided,
                        however, that except as otherwise provided herein or in
                        any Global Warrant Certificate, each Global Warrant
                        Certificate may be transferred only in whole and only to
                        the Depositary, to another nominee of the Depositary, to
                        a successor depository, or to a nominee of a successor
                        depository]. Upon any such registration of transfer, the
                        Company shall execute, and the Warrant Agent shall
                        countersign and deliver, as provided in Section 1.2, in
                        the name of the designated transferee a new Warrant
                        Certificate or Warrant Certificates of any authorized
                        denomination evidencing in the aggregate a like number
                        of unexercised Warrants.

                ( )     [If Warrants are not immediately detachable: After the
                        Detachment Date,] Upon surrender at the corporate office
                        of the Warrant Agent, properly endorsed or accompanied
                        by appropriate instruments of transfer and written
                        instructions for such exchange, all in form satisfactory
                        to the Company and the Warrant Agent, one or more
                        Warrant Certificates may be exchanged for one or more
                        Warrant Certificates in any other authorized
                        denominations; provided that such new Warrant
                        Certificate(s) evidence the same aggregate number of
                        Warrants as the Warrant Certificate(s) so surrendered.
                        Upon any such surrender for exchange, the Company shall
                        execute, and the Warrant Agent shall countersign and
                        deliver, as provided in Section 1.2, in the name of the
                        Holder of such Warrant Certificates, the new Warrant
                        Certificates.

                ( )     The Warrant Agent shall keep, at its corporate trust
                        office, books in which, subject to such reasonable
                        regulations as it may prescribe, it shall register
                        Warrant Certificates in accordance with Section 1.2 and
                        transfers, exchanges, exercises and cancellations of
                        outstanding Warrant Certificates. Whenever any Warrant
                        Certificates are surrendered for transfer or exchange in
                        accordance with this Section 1.3, an authorized officer
                        of the Warrant Agent shall manually countersign and
                        deliver the Warrant Certificates which the Holder making
                        the transfer or exchange is entitled to receive.

                ( )     No service charge shall be made for any transfer or
                        exchange of Warrant Certificates, but the Company may
                        require payment of a sum sufficient to cover any stamp
                        or other tax or other governmental charge that may be
                        imposed in connection with any such transfer or
                        exchange.

        Section 1.4     Lost, Stolen, Mutilated or Destroyed Warrant
Certificates. Upon receipt by the Company and the Warrant Agent of evidence
satisfactory to them of the ownership of and the loss, theft, destruction or
mutilation of any Warrant Certificate and of indemnity satisfactory to them and,
in the case of mutilation, upon surrender of such Warrant Certificate to the
Warrant Agent for cancellation, then, in the absence of notice to the Company or
the Warrant Agent that such Warrant Certificate has been acquired by a bona fide
purchaser, the Company shall execute, and an authorized officer of the Warrant
Agent shall manually countersign and deliver, in exchange for or in lieu of the
lost, stolen, destroyed or mutilated Warrant Certificate, a new Warrant
Certificate of the same tenor and for a like number of Warrants. No service
charge shall be made for any replacement of Warrant Certificates, but the
Company may require the payment of a sum sufficient to cover any stamp or other
tax or other governmental charge that may be


                                       5
<PAGE>   9


imposed in connection with any such exchange. To the extent permitted under
applicable law, the provisions of this Section 1.4 are exclusive with respect to
the replacement of mutilated, lost, stolen or destroyed Warrant Certificates and
shall preclude any and all other rights or remedies.


        Section 1.5     Cancellation of Warrant Certificates. Any Warrant
Certificate surrendered to the Warrant Agent for transfer, exchange or exercise
of the Warrants evidenced thereby shall be promptly canceled by the Warrant
Agent and shall not be reissued and, except as expressly permitted by this
Agreement, no Warrant Certificate shall be issued hereunder in lieu thereof. The
Warrant Agent shall deliver to the Company from time to time or otherwise
dispose of canceled Warrant Certificates in a manner satisfactory to the
Company. Any Warrant Certificate surrendered to the Company for transfer,
exchange or exercise of the Warrants evidenced thereby shall be promptly
delivered to the Warrant Agent and such transfer, exchange or exercise shall not
be effective until such Warrant Certificate has been received by the Warrant
Agent.

        Section 1.6     Treatment of Holders [If Warrants are to be issued in
Book-Entry Form and Beneficial Owners:] of Warrant Certificates. (a) The term
"Holder", as used herein, shall mean any person in whose name at the time any
Warrant Certificate shall be registered upon the books to be maintained by the
Warrant Agent for that purpose [If Warrants are not immediately detachable are
offered: or, prior to the Detachment Date, the person in whose name the Offered
Security to which such Warrant Certificate was initially attached is registered
upon the register relating to such Offered Securities. At all times prior to the
Detachment Date, the Company will, or will cause the registrar of the Offered
Securities to, make available to the Warrant Agent such information as to
holders of the Offered Securities as may be necessary to keep the Warrant
Agent's records current]. [If Warrants are to be issued in Book-Entry form: The
Holder of each Global Warrant Certificate shall initially be [Cede & Co.], a
nominee of the Depositary.]


                ( )     [If Warrants are to be issued in Book-Entry Form: (b)
                        The term "Beneficial Owner" as used herein shall mean
                        any person in whose name ownership of beneficial
                        interests in Warrants evidenced by a Global Warrant
                        Certificate is recorded in the records maintained by the
                        Depositary or its nominee, or by a Participant [If
                        Securities and Warrants that are not immediately
                        detachable are offered:, or, prior to the Detachment
                        Date, the person in whose name the Offered Security to
                        which such Warrant Certificate was initially attached is
                        registered upon the register relating to such Offered
                        Securities].]

                ( )     Every Holder [If Warrants are to be issued in
                        Book-Entry form: and every Beneficial Owner] consents
                        and agrees with the Company, the Warrant Agent and with
                        every subsequent Holder [If Warrants are to be issued in
                        Book-Entry form: and Beneficial Owner] that until the
                        Warrant Certificate is transferred on the books of the
                        Warrant Agent, the Company and the Warrant Agent may
                        treat the registered Holder of such Warrant Certificate
                        as the absolute owner of the Warrants evidenced thereby
                        for any purpose and as the person entitled to exercise
                        the rights attaching to the Warrants evidenced thereby,
                        any notice to the contrary notwithstanding.


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<PAGE>   10


                                   ARTICLE II
                EXERCISE PRICE, DURATION AND EXERCISE OF WARRANTS

        Section 2.1     Exercise Price. The exercise price of each Warrant shall
be [$________] (the "Exercise Price") [modify as appropriate to reflect terms of
offered Warrants].

        Section 2.2     Duration of Warrants. [Subject to the limitations set
forth herein,] Each Warrant may be exercised in whole but not in part [Unless
Warrants may be exercised on only one date:] on any Business Day (as defined
below) occurring during the period (the "Exercise Period") commencing on [its
date of issuance] [_________ __, ____] and ending at 5:00 P.M., New York time,]
on [__________ __, ____] (the "Expiration Date"). Each Warrant remaining
unexercised after 5:00 P.M., New York time, on the Expiration Date shall become
void, and all rights of the Holder under this Agreement shall cease.

                As used herein, the term "Business Day" means any day which is
not a Saturday or Sunday and is not a legal holiday or a day on which banking
institutions generally are authorized or obligated by law or regulation to close
in New York.

        Section 2.3     Exercise of Warrants.

                (a)     A Holder may exercise a Warrant by delivering, not later
                        than 5:00 P.M., New York time, on [Unless Warrants may
                        be exercised on only one date: any Business Day during
                        the Exercise Period (the "Exercise Date")] [If Warrants
                        may be exercised on only one date: the Expiration Date]
                        to the Warrant Agent at its corporate trust department
                        (i) the Warrant Certificate evidencing the Warrants to
                        be exercised, [If Warrants are to be issued in
                        Book-Entry form: and, in the case of a Global Warrant
                        Certificate, the Warrants to be exercised (the
                        "Book-Entry Warrants") free on the records of the
                        Depositary to an account of the Warrant Agent at the
                        Depositary designated for such purpose in writing by the
                        Warrant Agent to the Depositary from time to time,] (ii)
                        an election to purchase the Warrant Securities
                        ("Election to Purchase"), [For definitive Warrant
                        Certificates: properly completed and executed by the
                        Holder on the reverse of the Warrant Certificate] [If
                        Warrants are to be issued in Book-Entry form: or, in the
                        case of a Global Warrant Certificate, properly executed
                        by the Participant and substantially in the form
                        included on the reverse of each Warrant Certificate,]
                        and (iii) the Exercise Price for each Warrant to be
                        exercised in lawful money of the United States of
                        America by certified or official bank check or by bank
                        wire transfer in immediately available funds. If any of
                        (a) the Warrant Certificate [If Warrants are to be
                        issued in Book-Entry form: or the Book-Entry Warrants,]
                        (b) the Election to Purchase, or (c) the Exercise Price
                        therefor, is received by the Warrant Agent after 5:00
                        P.M., New York time, on [Unless Warrants may be
                        exercised on only one date: the specified Exercise Date,
                        the Warrants will be deemed to be received and exercised
                        on the Business Day next succeeding the Exercise Date.
                        If the date specified as the Exercise Date is not a
                        Business Day, the Warrants will be deemed to be received
                        and exercised on the next succeeding day which is a
                        Business Day. If the Warrants are received or deemed to
                        be received after] the Expiration Date, the exercise
                        thereof will be null and void and any funds delivered to
                        the Warrant Agent will be returned to the Holder [If
                        Warrants are to be issued in Book-Entry form: or
                        Participant, as the case may be,] as soon as


                                       7
<PAGE>   11


                        practicable. In no event will interest accrue on funds
                        deposited with the Warrant Agent in respect of an
                        exercise or attempted exercise of Warrants. The validity
                        of any exercise of Warrants will be determined by the
                        Warrant Agent in its sole discretion and such
                        determination will be final and binding upon the Holder
                        and the Company. Neither the Company nor the Warrant
                        Agent shall have any obligation to inform a Holder of
                        the invalidity of any exercise of Warrants. The Warrant
                        Agent shall deposit all funds received by it in payment
                        of the Exercise Price in the account of the Company
                        maintained with the Warrant Agent for such purpose and
                        shall advise the Company by telephone at the end of each
                        day on which funds for the exercise of the Warrants are
                        received of the amount so deposited to its account. The
                        Warrant Agent shall promptly confirm such telephonic
                        advice to the Company in writing.

                (b)     The Warrant Agent shall, by 11:00 A.M. on the Business
                        Day following the [Unless Warrants may be exercised on
                        only one date: Exercise Date of any Warrant] [If
                        Warrants may be exercised on only one date: Expiration
                        Date], advise the Company and the [Trustee under the
                        Indenture applicable to] [the transfer agent and
                        registrar in respect of] the Warrant Securities issuable
                        upon such exercise as to the number of Warrants
                        exercised in accordance with the terms and conditions of
                        this Agreement, the instructions of each Holder [If
                        Warrants are to be issued in Book-entry Form: or
                        Participant, as the case may be,] with respect to
                        delivery of the Warrant Securities issuable upon such
                        exercise, and the delivery of definitive Warrant
                        Certificates [If Warrants are to be issued in Book-Entry
                        form: or one or more Global Warrant Certificates, as
                        appropriate,] evidencing the balance, if any, of the
                        Warrants remaining after such exercise, and such other
                        information as the Company or such [Trustee] [transfer
                        agent and registrar] shall reasonably require.

                (c)     Subject to satisfaction of the conditions set forth in
                        Sections 2.3(a) and 2.3(b) above, the Company shall, by
                        5:00 P.M., New York time, on the third Business Day next
                        succeeding the [Unless Warrants may be exercised on only
                        one date: Exercise Date of any Warrant] [If Warrants may
                        be exercised on only one date: Expiration Date],
                        execute, issue and deliver to the Warrant Agent,
                        [pursuant to the Indenture applicable to the Warrant
                        Securities, the Warrant Securities, duly authenticated
                        by the Trustee of such Indenture and in authorized
                        denominations] [the Warrant Securities] to which such
                        Holder is entitled, in fully registered form, registered
                        in such name or names as may be directed by such Holder
                        [If Warrants are to be issued in Book-Entry form: or the
                        Participant, as the case may be]. Upon receipt of such
                        Warrant Securities, the Warrant Agent shall, by 5:00
                        P.M., New York time, on the fifth Business Day next
                        succeeding [Unless Warrants may be exercised on only one
                        date: such Exercise Date] [If Warrants may be exercised
                        on only one date: the Expiration Date], transmit such
                        Warrant Securities, to or upon the order of the Holder
                        [If Warrants are to be issued in Book-Entry form: or
                        Participant, as the case may be,] together with, or
                        preceded by the prospectus referred to in Section 5.6
                        hereof. The Company agrees that it


                                       8
<PAGE>   12


                        will provide such information and documents to the
                        Warrant Agent as may be necessary for the Warrant Agent
                        to fulfill its obligations hereunder.

                (d)     The accrual of [interest] [dividends], if any, on the
                        Warrant Securities issued upon the valid exercise of any
                        Warrant will be governed by the terms of the applicable
                        [Indenture] [certificate of designations and the Amended
                        and Restated Certificate of Incorporation of the Company
                        (the "Certificate of Incorporation")] and such Warrant
                        Securities. From and after the issuance of such Warrant
                        Securities, the former Holder of the Warrants exercised
                        will be entitled to the benefits of the [Indenture]
                        [certificate of designations and the Certificate of
                        Incorporation] under which such Warrant Securities are
                        issued and such former Holder's right to receive
                        payments of [principal of (and premium, if any) and
                        interest, if any, on] [dividends and any other amounts
                        payable in respect of] the Warrant Securities shall be
                        governed by, and shall be subject to, the terms and
                        provisions of such [Indenture] [certificate of
                        designations and Certificate of Incorporation] and the
                        Warrant Securities.

                (e)     Warrants may be exercised only in whole numbers of
                        Warrants. [Unless Warrants may be exercised on only one
                        date: If fewer than all of the Warrants evidenced by a
                        Warrant Certificate are exercised, a new Warrant
                        Certificate for the number of Warrants remaining
                        unexercised shall be executed by the Company and
                        countersigned by the Warrant Agent as provided in
                        Section 1.2 hereof, and delivered to the Holder at the
                        address specified on the books of the Warrant Agent or
                        as otherwise specified by such Holder.]

                (f)     The Company shall not be required to pay any stamp or
                        other tax or other governmental charge required to be
                        paid in connection with any transfer involved in the
                        issue of the Warrant Securities; and in the event that
                        any such transfer is involved, the Company shall not be
                        required to issue or deliver any Warrant Securities
                        until such tax or other charge shall have been paid or
                        it has been established to the Company's satisfaction
                        that no such tax or other charge is due.

        Section 2.4     Cancellation. The Warrant Agent and no one else shall
cancel all Warrant Certificates surrendered for registration of transfer,
exchange, exercise, replacement or cancellation and shall destroy cancelled
Warrant Certificates (subject to the record retention requirement of the
Exchange Act of 1934). Certification of the destruction of all cancelled Warrant
Certificates shall be delivered to the Company.

        [If Warrants for Class A Common Stock are offered: Section 2.5.
Adjustment Under Certain Circumstances. The Exercise Price and the number of
Warrant Securities purchasable upon the exercise of each Warrant shall be
subject to adjustment upon (i) the issuance of a stock dividend to the holders
of the outstanding shares of Warrant Securities or a combination, subdivision or
reclassification of the Warrant Securities; (ii) the issuance of rights,
warrants or options to all holders of the Warrant Securities entitling the
holders thereof to purchase Warrant Securities for an aggregate consideration
per share less than the current market price per share of the Warrant
Securities; or (iii) any distribution by the Company to the holders of the
Warrant


                                       9
<PAGE>   13


Securities of evidences of indebtedness of the Company or of assets (excluding
cash dividends or distributions payable out of consolidated earnings and earned
surplus and dividends or distributions referred to in (i) above); provided that
no such adjustment in the number of Warrant Securities purchasable upon exercise
of the Warrants will be required until cumulative adjustments require an
adjustment of at least 1% of such number. No fractional shares will be issued
upon exercise of Warrants, but the Company will pay the cash value of any
fractional shares otherwise issuable. The adjustments to be made under this
Section 2.5 shall be determined by the Warrant Agent and such determination
shall be final and binding upon the Holders and the Company.]

                                  ARTICLE III
                 OTHER PROVISIONS RELATING TO RIGHTS OF HOLDERS
                [IF WARRANTS ARE TO BE ISSUED IN BOOK-ENTRY FORM:
                       AND BENEFICIAL OWNERS] OF WARRANTS

        Section 3.1     No Rights as Holders of Warrant Securities Conferred by
Warrants or Warrant Certificates. No Warrant Certificate or Warrant evidenced
thereby shall entitle the Holder thereof to any of the rights of a holder of any
Warrant Securities, including, without limitation, [the right to receive the
payments of principal of (and premium, if any) and interest, if any, on Debt
Securities purchasable upon such exercise or to enforce any of the covenants in
the Indenture] [the right to receive dividends, if any, or payments upon the
liquidation, dissolution or winding up of the Company or to exercise voting
rights, if any].


        Section 3.2     Holder [If Warrants are to be issued in Book-Entry form:
and Beneficial Owner] of Warrant May Enforce Rights. Notwithstanding any of the
provisions of this Agreement, any Holder [If Warrants are to be issued in
Book-Entry form: and any Beneficial Owner] of any Warrant, without the consent
of the Warrant Agent or the Holder of any Warrant, may, on such Holder's [If
Warrants are to be issued in Book-Entry form: or Beneficial Owner's] own behalf
and for his own benefit, enforce, and may institute and maintain any suit,
action or proceeding against the Company to enforce, or otherwise in respect of,
such Holder's [If Warrants are to be issued in Book-Entry form: or Beneficial
Owner's] right to exercise the Warrants evidenced by any Warrant Certificate in
the manner provided in this Agreement and such Warrant Certificate.

                                   ARTICLE IV
                          CONCERNING THE WARRANT AGENT

        Section 4.1     Warrant Agent. The Company hereby appoints
[______________] as Warrant Agent of the Company in respect of the Warrants upon
the terms and subject to the conditions herein set forth, and [____________]
hereby accepts such appointment. The Warrant Agent shall have the powers and
authority granted to and conferred upon it hereby and such further powers and
authority to act on behalf of the Company as the Company may hereafter grant to
or confer upon it.

        Section 4.2     Limitations on Warrant Agent's Obligations. The Warrant
Agent accepts its obligations herein set forth upon the terms and conditions
hereof, including the following, to all of which the Company agrees and to all
of which the rights hereunder of the Holders from time to time shall be subject:


                                       10
<PAGE>   14


                (a)     Compensation and Indemnification. The Company agrees to
                        pay the Warrant Agent compensation to be agreed upon
                        with the Company for all services rendered by the
                        Warrant Agent and to reimburse the Warrant Agent for all
                        reasonable out-of-pocket expenses (including reasonable
                        counsel fees) incurred by the Warrant Agent in
                        connection with the services rendered by it hereunder.
                        The Company also agrees to indemnify the Warrant Agent
                        for, and to hold it harmless against, any loss,
                        liability or expense incurred without negligence, bad
                        faith or breach of this Agreement on the part of the
                        Warrant Agent, arising out of or in connection with its
                        acting as Warrant Agent hereunder.

                (b)     Agent for the Company. In acting in the capacity of
                        Warrant Agent under this Agreement, the Warrant Agent is
                        acting solely as agent of the Company and does not
                        assume any obligation or relationship of agency or trust
                        with any of the owners or holders of the Warrants except
                        as expressly set forth herein.

                (c)     Counsel. The Warrant Agent may consult with counsel
                        satisfactory to it (which may be counsel to the
                        Company), and the advice of such counsel shall be full
                        and complete authorization and protection in respect of
                        any action taken, suffered or omitted by it hereunder in
                        good faith and in accordance with the advice of such
                        counsel.

                (d)     Documents. The Warrant Agent shall be protected and
                        shall incur no liability for or in respect of any action
                        taken or thing suffered by it in reliance upon any
                        notice, direction, consent, certificate, affidavit,
                        statement or other paper or document reasonably believed
                        by it to be genuine and to have been presented or signed
                        by the proper parties.

                (e)     Certain Transactions. The Warrant Agent, and its
                        officers, directors and employees, may become the owner
                        of, or acquire any interest in, any Warrant, with the
                        same rights that it or they would have were it not the
                        Warrant Agent hereunder, and, to the extent permitted by
                        applicable law, it or they may engage or be interested
                        in any financial or other transaction with the Company
                        and may act on, or as a depositary, trustee or agent
                        for, any committee or body of holders of Warrants [If
                        Securities and Warrants are being offered together:,
                        Offered Securities] or Warrant Securities, or other
                        securities or obligations of the Company as freely as if
                        it were not the Warrant Agent hereunder. Nothing in this
                        Agreement shall be deemed to prevent the Warrant Agent
                        from acting as trustee under either Indenture.

                (f)     No Liability for Interest. The Warrant Agent shall not
                        be under any liability for interest on any monies at any
                        time received by it pursuant to any of the provisions of
                        this Agreement.

                (g)     No Liability for Invalidity. The Warrant Agent shall not
                        be under any responsibility with respect to the validity
                        or sufficiency of this Agreement or the execution and
                        delivery hereof (except the due execution and


                                       11
<PAGE>   15


                        delivery hereof by the Warrant Agent) or with respect to
                        the validity or execution of the Warrant Certificates
                        (except its countersignature thereon).

                (h)     No Responsibility for Recitals. The recitals contained
                        herein and in the Warrant Certificates (except as to the
                        Warrant Agent's countersignature thereon) shall be taken
                        as the statements of the Company and the Warrant Agent
                        assumes no responsibility hereby for the correctness of
                        the same.

                (i)     No Implied Obligations. The Warrant Agent shall be
                        obligated to perform such duties as are specifically set
                        forth herein and no implied duties or obligations shall
                        be read into this Agreement against the Warrant Agent.
                        The Warrant Agent shall not be under any obligation to
                        take any action hereunder which may tend to involve it
                        in any expense or liability, the payment of which within
                        a reasonable time is not, in its opinion, assured to it.
                        The Warrant Agent shall not be accountable or under any
                        duty or responsibility for the use by the Company of any
                        Warrant Certificate authenticated by the Warrant Agent
                        and delivered by it to the Company pursuant to this
                        Agreement or for the application by the Company of the
                        proceeds of the issue and sale, or exercise, of the
                        Warrants. The Warrant Agent shall have no duty or
                        responsibility in case of any default by the Company in
                        the performance of its covenants or agreements contained
                        herein or in any Warrant Certificate or in the case of
                        the receipt of any written demand from a Holder with
                        respect to such default, including, without limiting the
                        generality of the foregoing, any duty or responsibility
                        to initiate or attempt to initiate any proceedings at
                        law or otherwise or, except as provided in Section 5.3
                        hereof, to make any demand upon the Company.

        Section 4.3     Compliance With Applicable Laws. The Warrant Agent
agrees to comply with all applicable federal and state laws imposing obligations
on it in respect of the services rendered by it under this Agreement and in
connection with the Warrants, including (but not limited to) the provisions of
United States federal income tax laws regarding information reporting and backup
withholding. The Warrant Agent expressly assumes all liability for its failure
to comply with any such laws imposing obligations on it, including (but not
limited to) any liability for its failure to comply with any applicable
provisions of United States federal income tax laws regarding information
reporting and backup withholding.

        Section 4.4     Resignation and Appointment of Successor.

                (a)     The Company agrees, for the benefit of the Holders from
                        time to time, that there shall at all times be a Warrant
                        Agent hereunder until all the Warrants issued hereunder
                        have been exercised or have expired in accordance with
                        their terms, which Warrant Agent shall be a bank or
                        trust company organized under the laws of the United
                        States of America or one of the states thereof, which is
                        authorized under the laws of the jurisdiction of its
                        organization to exercise corporate trust powers, has a
                        combined capital and surplus of at least $50,000,000 and
                        has an office or an agent's office in the United States
                        of America.


                                       12
<PAGE>   16


                (b)     The Warrant Agent may at any time resign as such agent
                        by giving written notice to the Company of such
                        intention on its part, specifying the date on which it
                        desires such resignation to become effective; provided
                        that such date shall not be less than three months after
                        the date on which such notice is given, unless the
                        Company agrees to accept such notice less than three
                        months prior to such date of effectiveness. The Company
                        may remove the Warrant Agent at any time by giving
                        written notice to the Warrant Agent of such removal,
                        specifying the date on which it desires such removal to
                        become effective. Such resignation or removal shall take
                        effect upon the appointment by the Company, as
                        hereinafter provided, of a successor Warrant Agent
                        (which shall be a bank or trust company qualified as set
                        forth in Section 4.4(a)) and the acceptance of such
                        appointment by such successor Warrant Agent. The
                        obligation of the Company under Section 4.2(a) shall
                        continue to the extent set forth therein notwithstanding
                        the resignation or removal of the Warrant Agent.

                (c)     If at any time the Warrant Agent shall resign, or shall
                        cease to be qualified as set forth in Section 4.4(a), or
                        shall be removed, or shall become incapable of acting,
                        or shall be adjudged a bankrupt or insolvent, or shall
                        file a petition seeking relief under any applicable
                        Federal or State bankruptcy or insolvency law or similar
                        law, or make an assignment for the benefit of its
                        creditors or consent to the appointment of a receiver,
                        conservator or custodian of all or any substantial part
                        of its property, or shall admit in writing its inability
                        to pay or to meet its debts as they mature, or if a
                        receiver or custodian of it or of all or any substantial
                        part of its property shall be appointed, or if an order
                        of any court shall be entered for relief against it
                        under the provisions of any applicable Federal or State
                        bankruptcy or similar law, or if any public officer
                        shall have taken charge or control of the Warrant Agent
                        or of its property or affairs, for the purpose of
                        rehabilitation, conservation or liquidation, a successor
                        Warrant Agent, qualified as set forth in Section 4.4(a),
                        shall be appointed by the Company by an instrument in
                        writing, filed with the successor Warrant Agent. Upon
                        the appointment as herein provided of a successor
                        Warrant Agent and acceptance by the latter of such
                        appointment, the Warrant Agent so superseded shall cease
                        to be Warrant Agent under this Agreement.

                (d)     Any successor Warrant Agent appointed under this
                        Agreement shall execute, acknowledge and deliver to its
                        predecessor and to the Company an instrument accepting
                        such appointment, and thereupon such successor Warrant
                        Agent, without any further act, deed or conveyance,
                        shall become vested with all the authority, rights,
                        powers, trusts, immunities, duties and obligations of
                        such predecessor with like effect as if originally named
                        as Warrant Agent under this Agreement, and such
                        predecessor, upon payment of its charges and
                        disbursements then unpaid, shall thereupon become
                        obligated to transfer, deliver and pay over, and such
                        successor Warrant Agent shall be entitled to receive,
                        all monies, securities and other property on deposit
                        with or held by such predecessor, as Warrant Agent under
                        this Agreement.


                                       13
<PAGE>   17


                (e)     Any corporation into which the Warrant Agent may be
                        merged or converted or any corporation with which the
                        Warrant Agent may be consolidated, or any corporation
                        resulting from any merger, conversion or consolidation
                        to which the Warrant Agent shall be a party, or any
                        corporation to which the Warrant Agent shall sell or
                        otherwise transfer all or substantially all the assets
                        and business of the Warrant Agent, in each case provided
                        that it shall be qualified as set forth in Section
                        4.4(a), shall be the successor Warrant Agent under this
                        Agreement without the execution or filing of any paper
                        or any further act on the part of any of the parties to
                        this Agreement, including, without limitation, any
                        successor to the Warrant Agent first named above.

                                    ARTICLE V
                                  MISCELLANEOUS

        Section 5.1     Amendments.

                (a)     This Agreement and any Warrant Certificate may be
                        amended by the parties hereto by executing a
                        supplemental warrant agreement (a "Supplemental
                        Agreement"), without the consent of the Holder of any
                        Warrant, for the purpose of (i) curing any ambiguity, or
                        curing, correcting or supplementing any defective
                        provision contained herein, or making any other
                        provisions with respect to matters or questions arising
                        under this Agreement that is not inconsistent with the
                        provisions of this Agreement or the Warrant
                        Certificates, (ii) evidencing the succession of another
                        corporation to the Company and the assumption by any
                        such successor of the covenants of the Company contained
                        in this Warrant Agreement and the Warrants, (iii)
                        evidencing and providing for the acceptance of
                        appointment by a successor Warrant Agent with respect to
                        the Warrants, [If Warrants are to be issued in
                        Book-Entry form: (iv) evidencing and providing for the
                        acceptance of appointment by a successor Depositary with
                        respect to each Global Warrant Certificate, (v) issuing
                        definitive Warrant Certificates in accordance with
                        paragraph (b) of Section 1.3,] (vi) adding to the
                        covenants of the Company for the benefit of the Holders
                        or surrendering any right or power conferred upon the
                        Company under this Agreement, or (vii) amending this
                        Agreement and the Warrants in any manner that the
                        Company may deem to be necessary or desirable and that
                        will not adversely affect the interests of the Holders
                        in any material respect.

                (b)     The Company and the Warrant Agent may amend this
                        Agreement and the Warrants by executing a Supplemental
                        Agreement with the consent of the Holders of not fewer
                        than a majority of the unexercised Warrants affected by
                        such amendment, for the purpose of adding any provisions
                        to or changing in any manner or eliminating any of the
                        provisions of this Agreement or of modifying in any
                        manner the rights of the Holders under this Agreement;
                        provided, however, that, without the consent of each
                        Holder of Warrants affected thereby, no such amendment
                        may be made that (i) changes the Warrants so as to
                        reduce the [principal amount]


                                       14
<PAGE>   18


                        [number] of Warrant Securities purchasable upon exercise
                        of the Warrants or so as to increase the exercise price
                        [If Warrants for Class A Common Stock are offered:
                        (other than as provided by Section 2.3)], (ii) shortens
                        the period of time during which the Warrants may be
                        exercised, (iii) otherwise adversely affects the
                        exercise rights of the Holders in any material respect,
                        or (iv) reduces the number of unexercised Warrants the
                        consent of the Holders of which is required for
                        amendment of this Agreement or the Warrants.

        Section 5.2     Merger, Consolidation, Sale, Lease, Transfer or
Conveyance. The Company may consolidate or merge with or into any other
corporation or sell, lease, transfer or convey all or substantially all of its
assets to any other corporation, provided that (i) either (x) the Company is the
continuing corporation or (y) the corporation (if other than the Company) that
is formed by or results from any such consolidation or merger or that receives
such assets is a corporation organized and existing under the laws of the United
States of America or a state thereof and such corporation assumes the
obligations of the Company with respect to the performance and observance of all
of the covenants and conditions of this Agreement to be performed or observed by
the Company and (ii) the Company or such successor corporation, as the case may
be, must not immediately be in default under this Agreement. If at any time
there shall be any consolidation or merger or any sale, lease, transfer,
conveyance or other disposition of all or substantially all of the assets of the
Company, then the Company or such successor or assuming corporation, as the case
may be, shall make appropriate provision by amendment of this Agreement or
otherwise so that the holders of the Warrants then outstanding shall have the
right at any time thereafter, upon exercise of such Warrants, to receive the
kind and amount of shares of stock and other securities and property or assets
receivable upon such consolidation or merger or any sale, lease, transfer,
conveyance or other disposition as would be received by a holder of the number
of Warrant Securities issuable upon exercise of such Warrant immediately prior
to such consolidation or merger or sale, lease, transfer, conveyance or other
disposition, and in any such event the successor or assuming corporation shall
succeed to and be substituted for the Company, with the same effect as if it had
been named herein and in the Warrant Certificates as the Company; the Company
shall thereupon be relieved of any further obligation hereunder or under the
Warrants, and, in the event of any such sale, lease, transfer, conveyance (other
than by way of lease) or other disposition, the Company as the predecessor
corporation may thereupon or at any time thereafter be dissolved, wound up or
liquidated. Such successor or assuming corporation thereupon may cause to be
signed, and may issue either in its own name or in the name of the Company,
Warrant Certificates evidencing the Warrants not theretofore exercised, in
exchange and substitution for the Warrant Certificates theretofore issued. Such
Warrant Certificates shall in all respects have the same legal rank and benefit
under this Agreement as the Warrant Certificates evidencing the Warrants
theretofore issued in accordance with the terms of this Agreement as though such
new Warrant Certificates had been issued at the date of the execution hereof. In
any case of any such merger or consolidation or sale, lease, transfer,
conveyance or other disposition of all or substantially all of the assets of the
Company, such changes in phraseology and form (but not in substance) may be made
in the new Warrant Certificates, as may be appropriate.

        Section 5.3     Notices and Demands to the Company and Warrant Agent.
If the Warrant Agent shall receive any notice or demand addressed to the Company
by the Holder [If Warrants are to be issued in Book-Entry form: or a
Participant, as the case may be], the Warrant Agent shall promptly forward such
notice or demand to the Company.


                                       15
<PAGE>   19


        Section 5.4     Addresses. Any communications from the Company to the
Warrant Agent with respect to this Agreement shall be addressed to
[____________________, Attention: ________________________], and any
communications from the Warrant Agent to the Company with respect to this
Agreement shall be addressed to XO Communications, Inc., 11111 Sunset Hills
Drive, Reston, Virginia 20190, Attention: Corporate Secretary (or such other
address as shall be specified in writing by the Warrant Agent or by the Company,
as the case may be). The Company or the Warrant Agent shall give notice to the
Holders of Warrants by mailing written notice by first class mail, postage
prepaid, to such Holders as their names and addresses appear in the books and
records of the Warrant Agent [or, prior to the Detachment Date, on the register
of the Offered Securities].

        Section 5.5     GOVERNING LAW. THIS AGREEMENT AND EACH WARRANT
CERTIFICATE AND ALL RIGHTS HEREUNDER AND THEREUNDER AND PROVISIONS HEREOF AND
THEREOF SHALL BE GOVERNED BY, AND CONSTRUED IN ACCORDANCE WITH, THE LAWS OF THE
STATE OF NEW YORK (WITHOUT REFERENCE TO APPLICABLE CONFLICTS OF LAW PROVISIONS).

        Section 5.6     Delivery of Prospectus. The Company shall furnish to the
Warrant Agent sufficient copies of a prospectus relating to the Warrant
Securities deliverable upon exercise of Warrants and complying in all material
respects with the Securities Act of 1933, as amended (the "Prospectus"), and the
Warrant Agent agrees that upon the exercise of any Warrant, the Warrant Agent
shall deliver a Prospectus to the Holder of such Warrant, prior to or
concurrently with the delivery of the Warrant Securities issued upon such
exercise.

        Section 5.7     Obtaining of Governmental Approvals. The Company shall
from time to time take all action which may be necessary to obtain and keep
effective any and all permits, consents and approvals of governmental agencies
and authorities and securities acts filings under United States Federal and
state laws, which the Company may deem necessary or appropriate in connection
with the issuance, sale, transfer and delivery of the Warrants, the exercise of
the Warrants, the issuance, sale, transfer and delivery of the Warrant
Securities to be issued upon exercise of Warrants or upon the expiration of the
period during which the Warrants are exercisable.

        Section 5.8     Payment of Taxes. The Company will pay all stamp and
other duties, if any, to which, under the laws of the United States of America,
this Agreement or the original issuance of the Warrants may be subject.

        Section 5.9     Benefits of Warrant Agreement. Nothing in this Agreement
or any Warrant Certificate expressed or implied and nothing that may be inferred
from any of the provisions hereof or thereof is intended, or shall be construed,
to confer upon, or give to, any person or corporation other than the Company,
the Warrant Agent and their respective successors and assigns, [If Warrants are
to be issued in Book-Entry form: the Beneficial Owners] and the Holders any
right, remedy or claim under or by reason of this Agreement or any Warrant
Certificate or of any covenant, condition, stipulation, promise or agreement
hereof or thereof; and all covenants, conditions, stipulations, promises and
agreements contained in this Agreement or any Warrant Certificate shall be for
the sole and exclusive benefit of the Company and the Warrant Agent and their
respective successors and assigns and of the [If Warrants are to be issued in
Book-Entry form: Beneficial Owners and] Holders.


                                       16
<PAGE>   20


        Section 5.10    Headings. The descriptive headings of the several
Articles and Sections of this Agreement are inserted for convenience only and
shall not control or affect the meaning or construction of any of the provisions
hereof.

        Section 5.11    Severability. If any provision in this Agreement or in
any Warrant Certificate shall be invalid, illegal or unenforceable in any
jurisdiction, the validity, legality and enforceability of the remaining
provisions, or of such provisions in any other jurisdiction, shall not in any
way be affected or impaired thereby.

        Section 5.12    Counterparts. This Agreement may be executed in any
number of counterparts, each of which so executed shall be deemed to be an
original; but such counterparts shall together constitute but one and the same
instrument.

        Section 5.13    Inspection of Agreement. A copy of this Agreement shall
be available at all reasonable times at the principal corporate trust office of
the Warrant Agent and at the office of the Company for inspection by any Holder.
The Warrant Agent may require any such Holder to submit satisfactory proof of
ownership for inspection by it.





                                       17
<PAGE>   21


                IN WITNESS WHEREOF, the parties hereto have caused this
Agreement to be duly executed as of the day and year first above written.

                                             XO COMMUNICATIONS, INC.



                                             By:
                                                --------------------------------
                                             Authorized Officer



                                             [WARRANT AGENT]


                                             By:
                                                --------------------------------
                                             Authorized Officer








                                       18
<PAGE>   22


                                    EXHIBIT A

                          [FORM OF WARRANT CERTIFICATE]

                [Form of legend if Securities with Warrants that are not
immediately detachable or Warrants that are not immediately exercisable are
offered: [PRIOR TO _______________,] THIS WARRANT CERTIFICATE [(i) CANNOT BE
TRANSFERRED OR EXCHANGED UNLESS ATTACHED TO A [TITLE OF OFFERED SECURITY] AND
(II)] CANNOT BE EXERCISED IN WHOLE OR IN PART.]


                EXERCISABLE ONLY IF COUNTERSIGNED BY THE WARRANT
                            AGENT AS PROVIDED HEREIN.

                         Warrant Certificate evidencing

                              Warrants to Purchase

                          [Title of Warrant Securities]

                              as described herein.


                             XO COMMUNICATIONS, INC.

No. ___________                                         CUSIP No. _____________

           VOID AFTER [5:00 P.M.], NEW YORK TIME, ON [_______ __, ___]

                This certifies that [________________________] or registered
assigns is the registered holder of [Insert number initially issued] warrants to
purchase certain securities (the "Warrants"). Each Warrant entitles the holder
thereof, subject to the provisions contained herein and in the Warrant Agreement
referred to below, to purchase from XO Communications, Inc., a Delaware
corporation (the "Company"), [$_________ principal amount] [______] of the
Company's [title of Securities purchasable upon exercise of Warrants] [If
Warrants for Depositary Shares are to be offered:, each representing a [1/__th]
interest in a share of [title of securities represented by the Depositary
Shares]] (the "Warrant Securities" [If Warrants for Depositary Shares are to be
offered: , which term shall also refer, as appropriate, to such [title of
securities represented by the Depositary Shares]), [issued or to be issued under
the Indenture (as hereinafter defined)], at the Exercise Price set forth below.
The exercise price of each Warrant (the "Exercise Price") shall be [modify as
appropriate to reflect the terms of the offered Warrants].

                Subject to the terms of the Warrant Agreement, each Warrant
evidenced hereby may be exercised in whole but not in part at any time, as
specified herein, [Unless Warrants may be exercised on only one date: on any
Business Day (as defined below) occurring during the period (the "Exercise
Period") commencing on [the date of issuance thereof] [_______________ __, ____]
and ending at 5:00 P.M., New York time,] on [____________ __, ____] (the
"Expiration Date"). Each Warrant remaining unexercised after 5:00 P.M., New York
time, on

                                       19
<PAGE>   23


the Expiration Date shall become void, and all rights of the holder of this
Warrant Certificate evidencing such Warrant shall cease.

                The holder of the Warrants represented by this Warrant
Certificate may exercise any Warrant evidenced hereby by delivering, not later
than 5:00 P.M., New York time, on [Unless Warrants may be exercised on only one
date: any Business Day during the Exercise Period (the "Exercise Date")] [If
Warrants may be exercised on only one date: the Expiration Date] to [name of
Warrant Agent] (the "Warrant Agent", which term includes any successor warrant
agent under the Warrant Agreement described below) at its corporate trust
department at [___________________], (i) this Warrant Certificate [For Global
Warrant Certificate: and the Warrants to be exercised (the "Book-Entry
Warrants") free on the records of [The Depositary Trust Company] (the
"Depositary") to an account of the Warrant Agent at the Depositary designated
for such purpose in writing by the Warrant Agent to the Depositary], (ii) an
election to purchase ("Election to Purchase"), [For definitive Warrant
Certificates: properly executed by the holder hereof on the reverse of this
Warrant Certificate] [For Global Warrant Certificates: properly executed by the
institution in whose account the Warrant is recorded on the records of the
Depositary (the "Participant"), and substantially in the form included on the
reverse of hereof] and (iii) the Exercise Price for each Warrant to be exercised
in lawful money of the United States of America by certified or official bank
check or by bank wire transfer in immediately available funds. If any of (a)
this Warrant Certificate [For Global Warrant Certificates: or the Book-Entry
Warrants], (b) the Election to Purchase, or (c) the Exercise Price therefor, is
received by the Warrant Agent after 5:00 P.M., New York time, on [Unless
Warrants may be exercised on only one date: the specified Exercise Date, the
Warrants will be deemed to be received and exercised on the Business Day next
succeeding the Exercise Date. If the date specified as the Exercise Date is not
a Business Day, the Warrants will be deemed to be received and exercised on the
next succeeding day which is a Business Day. If the Warrants to be exercised are
received or deemed to be received after] the Expiration Date, the exercise
thereof will be null and void and any funds delivered to the Warrant Agent will
be returned to the holder as soon as practicable. In no event will interest
accrue on funds deposited with the Warrant Agent in respect of an exercise or
attempted exercise of Warrants. The validity of any exercise of Warrants will be
determined by the Warrant Agent in its sole discretion and such determination
will be final and binding upon the holder of the Warrants and the Company.
Neither the Warrant Agent nor the Company shall have any obligation to inform a
holder of Warrants of the invalidity of any exercise of Warrants. As used
herein, the term "Business Day" means any day which is not a Saturday or Sunday
and is not a legal holiday or a day on which banking institutions generally are
authorized or obligated by law or regulation to close in [__________].

                Warrants may be exercised only in whole numbers of Warrants.
[Unless Warrants may be exercised on only one date: If fewer than all of the
Warrants evidenced by this Warrant Certificate are exercised, a new Warrant
Certificate for the number of Warrants remaining unexercised shall be executed
by the Company and countersigned by the Warrant Agent as provided in Section 1.2
of the Warrant Agreement, and delivered to the holder of this Warrant
Certificate at the address specified on the books of the Warrant Agent or as
otherwise specified by such registered holder.]

                This Warrant Certificate is issued under and in accordance with
the Warrant Agreement, dated as of [___________ __, ____] (the "Warrant
Agreement"), between the Company and the Warrant Agent and is subject to the
terms and provisions contained in the Warrant Agreement, to all of which terms
and provisions the holder of this Warrant Certificate


                                       20
<PAGE>   24


[For Global Warrant Certificate: and the beneficial owners of the Warrants
represented by this Warrant Certificate] consent[s] by acceptance hereof. Copies
of the Warrant Agreement are on file and can be inspected at the above-mentioned
office of the Warrant Agent and at the office of the Company.

                [If the Warrant Securities are Debt Securities: The Warrant
Securities to be issued and delivered upon the exercise of the Warrants
evidenced by this Warrant Certificate will be issued under and in accordance
with the Indenture, dated as of [_________ __, ____] (the "Indenture"), between
the Company and [name of trustee], as trustee (together with any successor or
successors as such trustee, the "Trustee"), and will be subject to the terms and
provisions contained in the Warrant Securities and in the Indenture.] [If the
Warrant Securities are Preferred Stock: The Warrant Securities to be issued and
delivered upon the exercise of the Warrants evidenced by this Warrant
Certificate will be issued in accordance with the certificate of designations
and the Amended and Restated Certificate of Incorporation of the Company (the
"Certificate of Incorporation") and will be subject to the terms and provisions
contained in the certificate of designations and the Certificate of
Incorporation.] The accrual of [interest] [dividends], if any, on the Warrant
Securities issued upon the valid exercise of any Warrant will be governed by the
terms of the applicable [Indenture] [certificate of designations and the
Certificate of Incorporation] and such Warrant Securities. From and after the
issuance of such Warrant Securities, the former holder of the Warrants exercised
will be entitled to the benefits of the [Indenture] [certificate of designations
and the Certificate of Incorporation] under which such Warrant Securities are
issued and such former holder's right to receive payments of [principal of (and
premium, if any) and interest, if any, on] [dividends and any other amounts
payable in respect of] the Warrant Securities shall be governed by, and shall be
subject to, the terms and provisions of such [Indenture] [certificate of
designations and Certificate of Incorporation] and the Warrant Securities.
Copies of the [Indenture, including the form of the Warrant Securities, are on
file at the corporate trust office of the Trustee] [certificate of designations
and Certificate of Incorporation are available from the Company].

                [If the Warrant Securities are Class A Common Stock: The
Exercise Price and the number of Warrant Securities purchasable upon the
exercise of each Warrant shall be subject to adjustment upon (i) the issuance of
a stock dividend to the holders of the outstanding shares of Warrant Securities
or a combination, subdivision or reclassification of the Warrant Securities;
(ii) the issuance of rights, warrants or options to all holders of the Warrant
Securities entitling the holders thereof to purchase Warrant Securities for an
aggregate consideration per share less than the current market price per share
of the Warrant Securities; or (iii) any distribution by the Company to the
holders of the Warrant Securities of evidences of indebtedness of the Company or
of assets (excluding cash dividends or distributions payable out of consolidated
earnings and earned surplus and dividends or distributions referred to in (i)
above); provided that no such adjustment in the number of Warrant Securities
purchasable upon exercise of the Warrants will be required until cumulative
adjustments require an adjustment of at least 1% of such number. No fractional
shares will be issued upon exercise of Warrants, but the Company will pay the
cash value of any fractional shares otherwise issuable. The adjustments to be
made under this Section 2.3 shall be determined by the Warrant Agent and such
determination shall be final and binding upon the holders of the Warrants and
the Company.]

                [If Securities and Warrants are to be offered together: [If
Warrants are not immediately detachable: Prior to the Detachment Date,] The
Warrants represented by this Warrant Certificate may be exchanged or transferred
only together with the [title of Offered Security] (the "Offered Security") to
which the Warrants are attached, and only for the purpose


                                       21
<PAGE>   25


of effecting, or in conjunction with, an exchange or transfer of such Offered
Security. Additionally, [If Warrants are not immediately detachable: on or prior
to the Detachment Date,] each transfer of such Offered Security on the register
of the Offered Securities shall operate also to transfer the Warrants to which
such Offered Securities was initially attached. [If Warrants are not immediately
detachable: From and after the Detachment Date, the above provisions shall be of
no further force and effect.]] Upon due presentment for registration of transfer
or exchange of this Warrant Certificate at the corporate trust office of the
Warrant Agent, the Company shall execute, and the Warrant Agent shall
countersign and deliver, as provided in Section 1.2 of the Warrant Agreement, in
the name of the designated transferee one or more new Warrant Certificates of
any authorized denomination evidencing in the aggregate a like number of
unexercised Warrants, subject to the limitations provided in the Warrant
Agreement.

                Neither this Warrant Certificate nor the Warrants evidenced
hereby shall entitle the holder hereof or thereof to any of the rights of a
holder of the Warrant Securities, including, without limitation, [the right to
receive the payments of principal of (and premium, if any), and interest, if
any, on Debt Securities purchasable upon such exercise or to enforce any of the
covenants in the applicable Indenture] [the right to receive dividends, if any,
or payments upon the liquidation, dissolution or winding up of the Company or to
exercise voting rights, if any].

                The Warrant Agreement and this Warrant Certificate may be
amended as provided in the Warrant Agreement including, under certain
circumstances described therein, without the consent of the holder of this
Warrant Certificate or the Warrants evidenced thereby.

                THIS WARRANT CERTIFICATE AND ALL RIGHTS HEREUNDER AND UNDER THE
WARRANT AGREEMENT AND PROVISIONS HEREOF AND THEREOF SHALL BE GOVERNED BY, AND
CONSTRUED IN ACCORDANCE WITH, THE LAWS OF THE STATE OF NEW YORK (WITHOUT
REFERENCE TO APPLICABLE CONFLICTS OF LAW PROVISIONS).

                This Warrant Certificate shall not be entitled to any benefit
under the Warrant Agreement or be valid or obligatory for any purpose, and no
Warrant evidenced hereby may be exercised, unless this Warrant Certificate has
been countersigned by the manual signature of the Warrant Agent.




                                       22
<PAGE>   26


                IN WITNESS WHEREOF, the Company has caused this instrument to be
duly executed.

Dated as of ________ __, ____




                                             XO COMMUNICATIONS, INC.


                                             By:
                                                --------------------------------
                                             Authorized Officer



                                             [NAME OF WARRANT AGENT],
                                             as Warrant Agent


                                             By:
                                                --------------------------------
                                             Authorized Officer





<PAGE>   27


                                    [REVERSE]

                      INSTRUCTIONS FOR EXERCISE OF WARRANT

                To exercise the Warrants evidenced hereby, the holder [For
Global Warrant Certificate: or Participant] must, by 5:00 P.M., New York time,
on the specified Exercise Date, deliver to the Warrant Agent at its corporate
trust department, a certified or official bank check or a wire transfer in
immediately available funds, in each case payable to the Warrant Agent at
Account No. ____, in an amount equal to the Exercise Price in full for the
Warrants exercised. In addition, the Warrant holder [For Global Warrant
Certificates: or Participant] must provide the information required below and
deliver this Warrant Certificate to the Warrant Agent at the address set forth
below [For Global Warrant Certificates: and the Book-Entry Warrants to the
Warrant Agent in its account with the Depositary designated for such purpose].
This Warrant Certificate and the Election to Purchase must be received by the
Warrant Agent by 5:00 P.M., New York time, on the specified Exercise Date.

                              ELECTION TO PURCHASE
                    TO BE EXECUTED IF WARRANT HOLDER DESIRES
                    TO EXERCISE THE WARRANTS EVIDENCED HEREBY

                The undersigned hereby irrevocably elects to exercise, on
[__________, ____] (the "Exercise Date"), [_____________] Warrants, evidenced by
this Warrant Certificate, to purchase, [$_____________ principal amount]
[_________________] of the [title of Securities purchasable upon exercise of
Warrants] [If Warrants for Depositary Shares are to be offered: , each
representing a [1/__th] interest in a share of [title of securities represented
by the Depositary Shares]] (the "Warrant Securities") of XO Communications,
Inc., a Delaware corporation (the "Company"), and represents that on or before
the Exercise Date such holder has tendered payment for such Warrant Securities
by certified or official bank check or bank wire transfer in immediately
available funds to the order of the Company c/o [Name and address of Warrant
Agent], in the amount of [$_____________] in accordance with the terms hereof.
The undersigned requests that said [principal amount of] [number of] Warrant
Securities be in fully registered form, in the authorized denominations,
registered in such names and delivered, all as specified in accordance with the
instructions set forth below.



<PAGE>   28


                [Unless Warrants may be exercised on only one date: If said
[principal amount] [number] of Warrant Securities is less than all of the
Warrant Securities purchasable hereunder, the undersigned requests that a new
Warrant Certificate evidencing the remaining balance of the Warrants evidenced
hereby be issued and delivered to the holder of the Warrant Certificate unless
otherwise specified in the instructions below.]


Dated:  ______________ __, ____


                             Name
                                 -----------------------------------------------
                                 (Please Print)

                            / / / /- / / /- / / / / /

                             (Insert Social Security or Other Identifying Number
                             of Holder)
                                        ----------------------------------------

                             Address
                                           -------------------------------------

                                           -------------------------------------

                                           -------------------------------------


                             Signature
                                           -------------------------------------

                This Warrant may only be exercised by presentation to the
Warrant Agent at one of the following locations:

        By hand at



        By mail at



                The method of delivery of this Warrant Certificate is at the
option and risk of the exercising holder and the delivery of this Warrant
Certificate will be deemed to be made only when actually received by the Warrant
Agent. If delivery is by mail, registered mail with return receipt requested,
properly insured, is recommended. In all cases, sufficient time should be
allowed to assure timely delivery.

                (Instructions as to form and delivery of Warrant Securities
and/or Warrant Certificates)

        Name in which Warrant Securities are to be registered if other than in
the name of the registered holder of this Warrant Certificate:
                                                              ------------------


<PAGE>   29


                Address to which Warrant Securities are to be mailed if other
than to the address of the registered holder of this Warrant Certificate as
shown on the books of the Warrant Agent:


                                           -------------------------------------
                                           (Street Address)


                                           -------------------------------------
                                           (City and State) (Zip Code)

                [Except for Global Warrant Certificate: Name in which Warrant
Certificate evidencing unexercised Warrants, if any, are to be registered if
other than in the name of the registered holder of this Warrant Certificate:


---------------------------------


                Address to which certificate representing unexercised Warrants,
if any, are to be mailed if other than to the address of the registered holder
of this Warrant Certificate as shown on the books of the Warrant Agent:


                                                --------------------------------
                                                (Street Address)


                                                --------------------------------
                                                (City and State) (Zip Code)

Dated:
      ---------------------------


                                                --------------------------------
                                                Signature


<PAGE>   30


        ([Except for Global Warrant Certificate: Signature must conform in all
respects to the name of the holder as specified on the face of this Warrant
Certificate.] If Warrant Securities, or a Warrant Certificate evidencing
unexercised Warrants, are to be issued in a name other than that of the
registered holder hereof or are to be delivered to an address other than the
address of such holder as shown on the books of the Warrant Agent, the above
signature must be guaranteed by a member firm of a registered national stock
exchange, a member of the National Association of Securities Dealers, Inc., a
participant in the Security Transfer Agents Medallion Program or the Stock
Exchange Medallion Program, or by a commercial bank or trust company having an
office or correspondent in the United States.)

                                       SIGNATURE GUARANTEE



                                       Name of Firm
                                                   -----------------------------
                                       Address
                                              ----------------------------------
                                       Area Code and Number
                                                           ---------------------


                                       Authorized Signature
                                                           ---------------------

                                       Name
                                           -------------------------------------

                                       Title
                                            ------------------------------------


Dated:                  ,
        ----------------  ----

<PAGE>   31


                                   ASSIGNMENT

              (FORM OF ASSIGNMENT TO BE EXECUTED IF WARRANT HOLDER
                 DESIRES TO TRANSFER WARRANTS EVIDENCED HEREBY)
               FOR VALUE RECEIVED _________________ HEREBY SELLS,
          ASSIGNS AND TRANSFERS UNTO ________________________________

(Please print name and address
                                          --------------------------------------

                                          --------------------------------------

                                          --------------------------------------

                                          --------------------------------------



(Please insert social)
security or other identifying number)     --------------------------------------


                The rights represented by the within Warrant Certificate and
does hereby irrevocably constitute and appoint ____________ Attorney, to
transfer said Warrant Certificate on the books of the Warrant Agent with full
power of substitution in the premises.

Dated:
       ---------------------------------


                                               ---------------------------------
                                               Signature

(Signature must conform in all respects to the name of the holder as specified
on the face of this Warrant Certificate and must bear a signature guarantee by a
member firm of a registered national securities exchange, a member of the
National Association of Securities Dealers, Inc., a participant in the Security
Transfer Agents Medallion Program or the Stock Exchange Medallion Program, or by
a commercial bank or trust company having an office or correspondent in the
United States)


                                          SIGNATURE GUARANTEE

                                          Name of Firm
                                                      --------------------------

                                          Address
                                                      --------------------------

                                                      --------------------------

                                                      --------------------------


                                          Area Code and Number
                                                              ------------------


                                          Authorized Signature
                                                              ------------------

                                          Name
                                                --------------------------------
                                          Title
                                                --------------------------------

Dated:
      ---------------------------