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Exhibit 10.4 REGISTRATION RIGHTS AGREEMENT REGISTRATION RIGHTS AGREEMENT, dated as of June 23, 2004, between
Allegiance Telecom, Inc., (Allegiance), Allegiance Telecom Company Worldwide
(Allegiance Worldwide), Allegiance Telecom Liquidating Trust (the Trust, and
together with Allegiance and Allegiance Worldwide, the Allegiance Parties) and
XO Communications, Inc., a Delaware corporation (the Company). RECITALS WHEREAS, the Trust will acquire shares of Common Stock, par value
$.01 per share, of the Company (the Common Stock) to be issued pursuant to
that certain Asset Purchase Agreement (the Purchase Agreement), dated as of
February 18, 2004, between the Company and Allegiance as debtor-in-possession
and Allegiance Worldwide as debtor-in-possession; WHEREAS, pursuant to their Joint Plan of Reorganization of,
confirmed by order of the Bankruptcy Court for the Southern District of New York
on June 23, 2004, Allegiance and Allegiance Worldwide will transfer the Common
Stock to the Trust established thereunder to hold, sell or distribute to their
pre-petition creditors in accordance with its terms; and WHEREAS, the Company has agreed, pursuant to the Stipulation and
Order, entered on June 23, 2004, to grant the Trust registration rights upon the
terms and subject to the conditions hereinafter set forth; and NOW, THEREFORE, in consideration of the foregoing premises and
for other good and valuable consideration, the parties hereby agree as follows: SECTION 1. DEFINITIONS. As used in this Agreement, the following terms
have the respective meanings set forth below: COMMISSION: shall mean the Securities and Exchange Commission or
any other federal agency at the time administering the Securities Act; EXCHANGE ACT: shall mean the Securities Exchange Act of 1934, as
amended; MARKET STAND-OFF PERIOD: has the meaning defined in Section 5(a). PERSON: shall mean an individual, partnership,joint-stock
company, corporation, trust or unincorporated organization, and a government or
agency or political subdivision thereof; REGISTER, REGISTERED and REGISTRATION: shall be deemed to refer
to registration effected by preparing and filing a registration statement in
compliance with the Securities Act (and any post-effective amendments filed or
required to be filed) and the declaration or ordering of effectiveness of such
registration statement;




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REGISTRABLE SECURITIES: shall mean (A) shares of Common Stock
issued pursuant to the Purchase Agreement held by the Trust and (B) any stock of
the Company issued as a dividend or other distribution with respect to, or in
exchange for or in replacement of, the shares of the Common Stock referred to in
clause (A) held by the Trust; REGISTRATION EXPENSES: shall mean all expenses incurred by the
Company in compliance with Section 2(a), (b) and (c) hereof, including, without
limitation, all registration and filing fees, printing expenses, fees and
disbursements of counsel for the Company (other than blue sky fees and
expenses), and the expense of any special audits incident to or required by any
such registration (but excluding the compensation of regular employees of the
Company, which shall be paid in any event by the Company); SECURITY, SECURITIES: shall have the meaning set forth in Section
2(1) of the Securities Act; SECURITIES ACT: shall mean the Securities Act of 1933, as
amended; and SELLING EXPENSES: shall mean all underwriting discounts and
selling commissions applicable to the sale of Registrable Securities and all
fees and disbursements of counsel for the Trust, blue sky fees and expenses,
including blue sky fees and expenses of counsel for the Company. SECTION 2. INVESTMENT REPRESENTATIONS. (A) PRIVATE PLACEMENT EXEMPTION. Each of the Allegiance Parties
understands that the Common Stock has not been registered under the Securities
Act, nor qualified under any state securities laws, and that it has been offered
and sold pursuant to an exemption from such registration and qualification based
in part upon the representations of each of the Allegiance Parties contained
herein. (b) KNOWLEDGE OF OFFER. Each of the Allegiance Parties is
familiar with the business and operations of the Company and has been given the
opportunity to obtain from the Company all information that it has requested
regarding its business plans and prospects. (c) KNOWLEDGE AND EXPERIENCE; Ability to Bear Economic Risks.
Each of the Allegiance Parties has such knowledge and experience in financial
and business matters that it is capable of evaluating the merits and risks of
its investment in the Company. Each of the Allegiance Parties is able to bear
the economic risk of its investment in the Company (including a complete loss of
its investment). (d) LIMITATIONS ON DISPOSITION. Each of the Allegiance Parties
understands that it must bear the economic risk of this investment indefinitely
unless its Common Stock is registered pursuant to the Securities Act or an
exemption from such registration is available, and unless the disposition of
such Common Stock is qualified under applicable state securities laws or an
exemption from such qualification is available. Each of the Allegiance Parties
further understands that there is no assurance that any exemption from the
Securities Act will be available, or, if available, that such exemption will
allow it to transfer any or all of the Common Stock, in the amounts, or at the
time such Allegiance Party might propose.




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(e) INVESTMENT PURPOSE. Each of the Allegiance Parties is
acquiring the Common Stock solely, for its own account for investment and not
with a view toward the resale, transfer, or distribution thereof, nor with any
present intention of distributing the Common Stock. No other Person has any
right with respect to the Stock received by any of the Allegiance Parties, nor
has any of the Allegiance Parties agreed to give any Person any such shares of
Common Stock or right in the future. (f) ACCREDITED INVESTOR. Each of the Allegiance Parties is an
accredited investor as such term is defined in Rule 501 (a) promulgated under
the Securities Act and was not formed for the sole and specific purpose of
making the investment contemplated by this Agreement. (g) CAPACITY. Each of the Allegiance Parties has the requisite
corporate power and authority to enter into this Agreement and to consummate the
transactions contemplated by this Agreement. The execution and delivery of this
Agreement by each of the Allegiance Parties and the consummation by each such
party of the transactions contemplated hereby have been duly authorized by all
necessary corporate action on the part of each such Allegiance Party. This
Agreement has been duly executed and delivered by each of the Allegiance Parties
and, assuming this Agreement constitutes the valid and binding obligation of the
Company, constitutes the valid and binding obligation of each of the Allegiance
Parties, enforceable against each such Allegiance Party in accordance with its
terms. SECTION 3. REGISTRATION RIGHTS. (a) FORM S-3 REGISTRATION. The Company represents and warrants
that it currently qualifies for registration on Form S-3 for secondary sales,
and covenants and agrees that, during the Shelf Period (as defined below), it
will continue to file all reports required by the Exchange Act necessary to
maintain such qualification. So long as the Company remains qualified for the
use of Form S-3 for secondary sales, the Company shall use its best efforts to
prepare, file and cause the effectiveness of a Registration Statement on Form
S-3 for all of the Registrable Securities (provided that the Trust shall have
informed the Company in writing of its intended method of disposition of the
Registrable Securities), and (provided that Rule 415, or any successor rule
under the Securities Act, at the time permits an offering on a continuous or
delayed basis, and provided further that applicable rules under the Securities
Act governing the obligation to file a post-effective amendment permit, in lieu
of filing a post-effective amendment which (y) includes any prospectus required
by Section 10(a) of the Securities Act or (z) reflects facts or events
representing a material or fundamental change in the information set forth in
the registration statement, the incorporation by reference of information
required to be included in (y) and (z) above to be contained in periodic reports
filed pursuant to Section 12 or 15(d) of the Exchange Act in the registration
statement) the Company will use its best efforts to maintain the effectiveness
of such registration for three years following the Closing under the Asset
Purchase Agreement, or until such time as all such Registrable Securities are
sold or are otherwise eligible for sale without registration (such period, the
Shelf Period); provided, however, that (A) such three year period shall be
extended for a period of time equal to the period during which the Trust
refrains from selling any securities included in such registration in accordance
with provisions in Section 3 and 5(a) and for any time necessary to make
corrections to the applicable prospectus pursuant to Section 6(b); provided that
the Company shall not be obligated to effect, or take any action to effect or




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maintain the effectiveness of, any such registration pursuant to this Section
3(a): (i) during a Market Stand-Off Period; or (ii) In any jurisdiction in which the Company would be
required to execute a general consent to service of process in
effecting such registration, qualification or compliance, unless the
Company is already subject to service in such jurisdiction and except
as may be required by the Securities Act or applicable rules or
regulations thereunder. (b) S-1 REGISTRATION. In the event that the Company becomes
ineligible to use Form S-3 and cannot certify its reasonable, good faith
expectation that it will regain such eligibility within 90 days, upon request of
the Trust, the Company will as soon as practicable, use its best efforts to
effect a registration of sales of the Registrable Securities on another
appropriate form (including, without limitation, the execution of an undertaking
to file post-effective amendments, appropriate qualification under applicable
blue sky or other state securities laws and appropriate compliance with
applicable regulations issued under the Securities Act) as would permit or
facilitate the sale and distribution of all or such portion of such Registrable
Securities in such manner as the Trust shall specify to the Company in writing;
provided that the Company shall not be obligated to effect, or take any action
to effect, any such registration pursuant to this Section 3(b): (i) In any particular jurisdiction in which the Company would
be required to execute a general consent to service of process in
effecting such registration, qualification or compliance, unless the
Company is already subject to service in such jurisdiction and except
as may be required by the Securities Act or applicable rules or
regulations thereunder; or (ii) After the Company has effected three (3) such
registrations pursuant to this Section 3(b) and such registrations have
been declared or ordered effective and the sales of such Registrable
Securities shall have closed; or (iii) During a Market Stand-Off Period. (iv) The Company will use its best efforts to keep such
registration effective for a period of one hundred twenty (120) days or
until the Trust has completed the distribution described in the
registration statement relating thereto, whichever first occurs;
provided however, that such 120-day period shall be extended for a
period of time equal to the period during which the Trust refrains from
selling any securities included in such registration in accordance with
provisions in Section 3 and 5(a) and for any time necessary to make
corrections to the applicable prospectus pursuant to Section 6(b).
Notwithstanding the foregoing, if the Companys ineligibility to use
Form S-3 for secondary offerings is the result of a breach of the first
sentence of Section 3(a), and (provided that Rule 415, or any successor
rule under the Securities Act, at the time permits an offering on a
continuous or delayed basis) the Company will use its best efforts to
maintain the effectiveness of such registration for three years
following the Closing under the Asset Purchase Agreement, or until such
time as all such Registrable Securities are sold or are otherwise




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eligible for sale without registration (such period, the Shelf
Period); provided that the Company shall not be obligated to effect,
or take any action to effect or maintain the effectiveness of, any such
registration under the conditions set forth in Section 3(a)(i) and
(ii); provided, further, that (A) such three year period shall be
extended for a period of time equal to the period during which the
Trust refrains from selling any securities included in such
registration in accordance with provisions in Section 3 and 5(a) and
for any time necessary to make corrections to the applicable prospectus
pursuant to Section 6(b). (v) The registration statement filed pursuant to the request
of the Trust may, subject to the provisions of Section 3(b)(vii) below,
include other securities of the Company which are held by Persons (not
including the Company or any of its officers, directors or other
entities controlled by Mr. Carl Icahn) who, by virtue of agreements
with the Company, are entitled to include their securities in any such
registration (Other Stockholders). (vi) If Other Stockholders request such inclusion, the Trust
shall offer to include the securities of such Other Stockholders in the
underwriting and may condition such offer on their acceptance of the
further applicable provisions of this Section 3. The Trust and the
Company shall (together with all Other Stockholders proposing to
distribute their securities through such underwriting) enter into an
underwriting agreement in customary form with the representative of the
underwriter or underwriters selected for such underwriting by the Trust
and reasonably acceptable to the Company. Notwithstanding any other
provision of this Section 3(b), if the representative advises the Trust
in writing that marketing factors require a limitation on the number of
 shares to be underwritten, the number of shares included in the
registration by the Trust and each Other Stockholder shall be reduced
on a pro rata basis (based on the number of shares held by the Trust
and Other Stockholder), by such minimum number of shares as is
necessary to comply with such request. No Registrable Securities or any
other securities excluded from the underwriting by reason of the
underwriters marketing limitation shall be included in such
registration. If any Other Stockholder who has requested inclusion in
such registration as provided above disapproves of the terms of the
underwriting, such Person may elect to withdraw therefrom by written
notice to the Company, the underwriter and the Trust. The securities so
withdrawn shall also be withdrawn from registration. If the underwriter
has not limited the number of Registrable Securities or other
securities to be underwritten, the Company and officers and directors
of the Company may include its or their securities for its or their own
account in such registration if the representative so agrees and if the
number of Registrable Securities and other securities which would
otherwise have been included in such registration and underwriting will
not thereby be limited. (c) EXPENSES OF REGISTRATION. All Registration Expenses incurred
in connection with any registration, qualification or compliance pursuant to
this Section 3 shall be borne by the Company. All Selling Expenses shall be
borne by the Trust. (d) TERMINATION. The registration rights set forth in this




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Section 3 shall not be available to the Trust if, (i) in the opinion of counsel
to the Company, all of the Registrable Securities then owned by the Trust could
be sold in any 90-day period pursuant to Rule 144 (without giving effect to the
provisions of Rule 144(k)) or (ii) all of the Registrable Securities held by the
Trust have been sold in a registration pursuant to the Securities Act or
pursuant to Rule 144. SECTION 4. RULE 144 REPORTING. With a view to making available the
benefits of certain rules and regulations of the Commission which may permit the
sale of restricted securities to the public without registration, the Company
agrees to: (a) make and keep public information available as those terms are
understood and defined in Rule 144 under the Securities Act (Rule 144), at all
times; (b) use its best efforts to file with the Commission in a timely
manner all reports and other documents required of the Company under the
Securities Act and the Exchange Act at any time after it has become subject to
such reporting requirements; and (c) so long as the Trust owns any Registrable Securities, furnish
to the Trust upon request, a written statement by the Company as to its
compliance with the reporting requirements of Rule 144, and of the Securities
Act and the Exchange Act, a copy of the most recent annual or quarterly report
of the Company, and such other reports and documents so filed as the Trust may
reasonably request in availing itself of any rule or regulation of the
Commission allowing the Trust to sell any such securities without registration. SECTION 5. COVENANTS. (a) MARKET STAND-OFF. Each of the Allegiance Parties agrees that
it will not sell or otherwise transfer or dispose of any Registrable Securities
held by such Allegiance Party during the period referred to below (a Market
Stand-Off Period): (i) in connection with an underwritten offering of equity or
equity-linked securities (such as convertible securities or warrants,
but not including a registration of securities in a Rule 145
transaction or with respect to an employee benefit plan)) of the
Company, as requested by the Company and the Companys underwriter for
a period starting with the date of filing of, and ending on the date 90
days immediately following the effective date of, the applicable
registration statement, provided that (i) the executive officers (as
defined for purposes of Rule 16a-1 under the Exchange Act) and all the
directors of the Company enter into similar agreements and (ii) the
Company is actively employing in good faith all reasonable efforts to
cause such registration statement to become effective. The Company may
only delay or suspend an offering pursuant to this Section 5(a)(i) for
a period of not more than sixty (60) days, if a filing of any other
registration statement is not made within that period and the Company
may only exercise that right once in any twelve (12) month period. (ii) if the Company determines in its good faith judgment that
the filing of a registration statement under Section 3 or the use of
any related prospectus would require the disclosure of material
information that the Company has a bona fide business purpose for




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preserving as confidential or the disclosure of which would impede the
Companys ability to consummate a significant transaction, and that the
Company is not otherwise required by applicable securities laws or
regulations to disclose, upon written notice of such determination by
the Company, until the date upon which the Company notifies the
Allegiance Parties in writing that suspension of such rights for the
grounds set forth in this Section 5(a)(ii) is no longer necessary,
provided that (A) (x) the Company may not declare more than three
Market Stand-Off Periods in any 12month period pursuant to this clause
(ii), (y) the aggregate Market Stand-Off Period pursuant to this clause
(ii) in any 12-month period shall not exceed 90 days, and (z) the
Market Stand-Off Period pursuant to this clause (ii) may not be used to
extend the 90-day period contemplated by clause (i) above to a total
continuous Market Stand-Off period exceeding 150 days in the aggregate
and (B) unless the registration statement in question is a shelf
registration statement for a delayed or continuous offering, then if
the Company declares a Market Stand-Off period during the first 60 days
following the effectiveness thereof, each day of such Market Stand-Off
period will be counted as 1 1/2 days against the 90-day period
contemplated by clause (i) above. The Company agrees to give such
notice as promptly as practicable following the date that such
suspension of rights is no longer necessary. (b) SUPPLEMENTAL UNDERTAKING. If requested by the underwriters,
the Allegiance Parties shall execute a separate agreement to the effect set
forth in Section 5(a)(i). The Company may impose stop-transfer instructions with
respect to the shares (or securities) subject to the foregoing restriction until
the end of said 180-day period. The provisions of this Section 5(b) shall be
binding upon any transferee who acquires Registrable Securities. (c) RESTRICTIONS ON TRANSFER. Each of the Allegiance Parties
agrees that none of the Registrable Securities shall be transferred, sold,
assigned, pledged, hypothecated, or otherwise disposed or encumbered, either
voluntarily or involuntarily, directly or indirectly, except pursuant to an
effective registration under the Securities Act, or in a transaction which, in
the opinion of counsel experienced in such matters and reasonably satisfactory
to the Company, qualifies as an exempt transaction under the Securities Act and
the rules and regulations promulgated thereunder. (d) INFORMATION SUPPLIED BY THE TRUST. The Trust shall furnish to
the Company information regarding itself and the distribution proposed by such
Trust as the Company may reasonably request in writing and as shall be
reasonably required in connection with any registration, qualification or
compliance referred to in Section 3. SECTION 6. REGISTRATION PROCEDURES. In the case of each registration
effected by the Company pursuant to Section 3, the Company will keep the Trust
advised in writing as to the initiation of each registration and as to the
completion thereof. At its own expense, the Company will: (a) furnish such number of prospectuses and other documents
incident thereto as the Trust from time to time may reasonably request; (b) notify the Trust when a prospectus relating to the
registration statement is required to be delivered under the Securities Act of
the happening of any event as a result of which the prospectus included in such
registration statement, as then in effect, includes an untrue statement of a




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material fact or omits to state a material fact required to be stated therein or
necessary to make the statements therein not misleading in the light of the
circumstances then existing; and (c) furnish, on the date that such Registrable Securities are
delivered to the underwriters for sale, if such securities are being sold
through underwriters or, if such securities are not being sold through
underwriters, on the date that the registration statement with respect to such
securities becomes effective, (1) an opinion, dated as of such date, of the
counsel representing the Company for the purposes of such registration, in form
and substance as is customarily given to underwriters in an underwritten public
offering and reasonably satisfactory the Trust, addressed to the underwriters,
if any, and to the Trust and (2) a letter, dated as of such date, from the
independent certified public accountants of the Company, in form and substance
as is customarily given by independent certified public accountants to
underwriters in an underwritten public offering and reasonably satisfactory to
the Trust, addressed to the underwriters, if any, and if permitted by applicable
accounting standards, to the Trust. SECTION 7. INDEMNIFICATION. (a) The Company will indemnify the Trust, as applicable, each of
its officers, directors and partners, and each Person controlling the Trust,
with respect to each registration which has been effected pursuant to Section 3,
and each underwriter, if any, and each person who controls any underwriter,
against all claims, losses, damages and liabilities (or actions in respect
thereof) arising out of or based on any untrue statement (or alleged untrue
statement) of a material fact contained in any prospectus, offering circular or
other document (including any related registration statement, notification or
the like) incident to any such registration, qualification or compliance, or
based on any omission (or alleged omission) to state therein a material fact
required to be stated therein or necessary to make the statements therein not
misleading, or any violation by the Company of the Securities Act or the
Exchange Act or any rule or regulation thereunder applicable to the Company and
relating to action or inaction required of the Company in connection with any
such registration, qualification or compliance, and will reimburse the Trust,
each of its officers, directors and partners, and each Person controlling the
Trust, each such underwriter and each Person who controls any such underwriter,
for any legal and any other expenses reasonably incurred in connection with
investigating and defending any such claim, loss, damage, liability or action,
provided that the Company will not be liable in any such case to the extent that
any such claim, loss, damage, liability or expense arises out of or is based on
any untrue statement or omission based upon written information furnished to the
Company by the Trust or underwriter and stated to be specifically for use
therein. (b) The Trust will indemnify the Company, each of its directors
and officers and each underwriter, if any, of the Companys securities covered
by such a registration statement, each person who controls the Company or such
underwriter, each Other Stockholder and each of their officers, directors, and
partners, and each person controlling such Other Stockholder against all claims,
losses, damages and liabilities (or actions in respect thereof) arising out of
or based on any untrue statement (or alleged untrue statement) of a material
fact contained in any such registration statement, prospectus, offering circular
or other document made by the Trust, or any omission (or alleged omission) to
state therein a material fact required to be stated therein or necessary to make




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the statements by the Trust therein not misleading, and will reimburse the
Company and such Other Stockholders, directors, officers, partners, persons,
underwriters or control persons for any legal or any other expenses reasonably
incurred in connection with investigating or defending any such claim, loss,
damage, liability or action, in each case to the extent, but only to the extent,
that such untrue statement (or alleged untrue statement) or omission (or alleged
omission) is made in such registration statement, prospectus, offering circular
or other document in reliance upon and in conformity with written information
furnished to the Company by the Trust and stated to be specifically for use
therein; provided, however, that the obligations of the Trust hereunder shall be
limited to an amount equal to the net proceeds to the Trust of securities sold
as contemplated herein. (c) Each party entitled to indemnification under this Section 7
(the Indemnified Party) shall give notice to the party required to provide
indemnification (the Indemnifying Party) promptly after such Indemnified Party
has actual knowledge of any claim as to which indemnity may be sought, and shall
permit the Indemnifying Party to assume the defense of any such claim or any
litigation resulting therefrom; provided that counsel for the Indemnifying
Party, who shall conduct the defense of such claim or any litigation resulting
therefrom, shall be approved by the Indemnified Party (whose approval shall not
unreasonably be withheld) and the Indemnified Party may participate in such
defense at such partys expense (unless the Indemnified Party shall have
reasonably concluded that there may be a conflict of interest between the
Indemnifying Party and the Indemnified Party in such action, in which case the
fees and expenses of counsel shall be at the expense of the Indemnifying Party),
and provided further that the failure of any Indemnified Party to give notice as
provided herein shall not relieve the Indemnifying Party of its obligations
under this Section 7 unless the Indemnifying Party is materially prejudiced
thereby. No Indemnifying Party, in the defense of any such claim or litigation
shall, except with the consent of each Indemnified Party, consent to entry of
any judgment or enter into any settlement which does not include as an
unconditional term thereof the giving by the claimant or plaintiff to such
Indemnified Party of a release from all liability in respect to such claim or
litigation. Each Indemnified Party shall furnish such information regarding
itself or the claim in question as an Indemnifying Party may reasonably request
in writing and as shall be reasonably required in connection with the defense of
such claim and litigation resulting therefrom. (d) If the indemnification provided for in this Section 7 is held
by a court of competent jurisdiction to be unavailable to an Indemnified Party
with respect to any loss, liability, claim, damage or expense referred to
herein, then the Indemnifying Party, in lieu of indemnifying such Indemnified
Party hereunder, shall contribute to the amount paid or payable by such
Indemnified Party as a result of such loss, liability, claim, damage or expense
in such proportion as is appropriate to reflect the relative fault of the
Indemnifying Party on the one hand and of the Indemnified Party on the other in
connection with the statements or omissions which resulted in such loss,
liability, claim, damage or expense, as well as any other relevant equitable
considerations. The relative fault of the Indemnifying Party and of the
Indemnified Party shall be determined by reference to, among other things,
whether the untrue (or alleged untrue) statement of a material fact or the
omission (or alleged omission) to state a material fact relates to information
supplied by the Indemnifying Party or by the Indemnified Party and the parties
relative intent, knowledge, access to information and opportunity to correct or
prevent such statement or omission.




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(e) Notwithstanding the foregoing, to the extent that the
provisions on indemnification and contribution contained in the underwriting
agreement entered into in connection with any underwritten public offering
contemplated by this Agreement are in conflict with the foregoing provisions,
the provisions in such underwriting agreement shall be controlling. (i) The foregoing indemnity agreement of the Company and the
Trust is subject to the condition that, insofar as they relate to any
loss, claim, liability or damage arising out of a statement made in or
omitted from a preliminary prospectus but eliminated or remedied in the
amended prospectus on file with the Commission at the time the
registration statement in question becomes effective or the amended
prospectus filed with the Commission pursuant to Commission Rule 424(b)
(the Final Prospectus), such indemnity or contribution agreement
shall not inure to the benefit of any underwriter or the Trust if a
copy of the Final Prospectus was furnished to the underwriter and was
not furnished to the Person asserting the loss, liability, claim or
damage at or prior to the time such action is required by the
Securities Act. SECTION 8. MISCELLANEOUS. (a) DIRECTLY
OR INDIRECTLY. Where any provision in this Agreement refers to action to be taken by any Person, or which such Person is prohibited
from taking, such provision shall be applicable whether such action is taken
directly or indirectly by such Person. (b) GOVERNING LAW. This Agreement shall be governed by and
construed in accordance with the laws of the State of New York applicable to
contracts made and to be performed entirely within such State. (c) SECTION HEADINGS. The headings of the sections and
subsections of this Agreement are inserted for convenience only and shall not be
deemed to constitute a part thereof. (d) NOTICES. (i) All communications under this Agreement shall be in
writing and shall be delivered by hand or facsimile or mailed by
overnight courier or by registered or certified mail, postage prepaid: (1) if to the Company, to: 11111 Sunset Hills Road
Reston, Virginia 20190 (facsimile: (703) 547-2479), Attention:
General Counsel, or at such other address as it may have
furnished in writing to the Allegiance Parties, with a copy to
Willkie Farr & Gallagher LLP, 787 Seventh Avenue, New York, NY
10019 (facsimile: (212) 728-8111), Attention: Bruce R. Kraus,
Esq. (2) if to the Allegiance or Allegiance Worldwide, to
[]: or at such other address or facsimile
number as may have been furnished the Company in writing. (3) if to the Trust, to: [], or
at such other address or facsimile number as may have been
furnished the Company in writing.




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(ii) Any notice so addressed shall be deemed to be given: if
delivered by hand or facsimile, on the date of such delivery; if mailed
by overnight courier, on the first business day following the date of
such mailing; and if mailed by registered or certified mail, on the
third business day after the date of such mailing. (e) REPRODUCTION OF DOCUMENTS. This Agreement and all documents
relating thereto, including, without limitation, any consents, waivers and
modifications which may hereafter be executed may be reproduced by the
Allegiance Parties by any photographic, photostatic, microfilm, microcard,
miniature photographic or other similar process and the Allegiance Parties may
destroy any original document so reproduced. The parties hereto agree and
stipulate that any such reproduction shall be admissible in evidence as the
original itself in any judicial or administrative proceeding (whether or not the
original is in existence and whether or not such reproduction was made by the
Allegiance Parties in the regular course of business) and that any enlargement,
facsimile or further reproduction of such reproduction shall likewise be
admissible in evidence. (f) SUCCESSORS AND ASSIGNS. Except as otherwise provided in this
Agreement, no party hereto shall assign this Agreement or any rights or
obligations hereunder without the prior written consent of the other parties
hereto and any such attempted assignment without such prior written consent
shall be void and of no force and effect. This Agreement shall inure to the
benefit of and be binding upon the successors and assigns of each of the
parties. The rights of the Trust under this Agreement may be assigned to no more
than 10 purchasers of Registrable Securities, provided that each such assignee
shall promptly execute and deliver to the Company a written undertaking assuming
the obligations of the Trust under this Agreement, and provided further that if,
at any time, there is more than one holder of Registrable Securities, demands,
requests and determinations otherwise made by the Trust shall be made by the
holders of a majority of the Registrable Securities at the time outstanding. (g) ENTIRE AGREEMENT; AMENDMENT AND WAIVER. This Agreement
constitutes the entire understanding of the parties hereto and supersedes all
prior understanding among such parties. This Agreement may be amended, and the
observance of any term of this Agreement may be waived, with (and only with) the
written consent of the Company. and the Trust. (h) SEVERABILITY. In the event that any part or parts of this
Agreement shall be held illegal or unenforceable by any court or administrative
body of competent jurisdiction, such determination shall not affect the
remaining provisions of this Agreement which shall remain in full force and
effect. (i) COUNTERPARTS. This Agreement may be executed in two or more
counterparts (including by facsimile), each of which shall be deemed an original
and all of which together shall be considered one and the same agreement. IN WITNESS WHEREOF, the undersigned have executed this Agreement
as of the date first set forth above. XO COMMUNICATIONS, INC.




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By:
    /s/ William GarrahanName:
    William GarrahanTitle:
    Acting CFO ALLEGIANCE TELECOM, INC.,
as debtor-in-possession By:
    
    /s/ Mark B. TresnowskiName: Mark B. TresnowskiTitle: Executive Vice President
 ALLEGIANCE TELECOM COMPANY WORLDWIDE,
as debtor-in-possession By:
    
    /s/ Mark B. TresnowskiName: Mark B. TresnowskiTitle: Executive Vice President
 ALLEGIANCE TELECOM LIQUIDATING TRUST By:
    
    /s/ Eugene I. DavisName: Eugene I. DavisTitle: Plan Administrator