<PAGE>

                                                                   EXHIBIT 10.66


                              AMENDMENT AGREEMENT

This Amendment Agreement ("Agreement") is dated as of 15th day of June, 1999
(the "Effective Date"), and is made by and among Equalnet Communications Corp.,
a Texas corporation, hereinafter referred to as "Equalnet" or "Company", and
Willis Group, LLC, a Texas limited liability company, hereinafter referred to as
"Holder."

WHEREAS, Holder is the holder of record of 1,982 shares of Series D Preferred
Stock; and

WHEREAS, pursuant to the Note Purchase and Exchange Agreement, dated as of July
31, 1998, by and between the Company and the Holder (the "Purchase and Exchange
Agreement"), the Company issued to the Holder shares (the "Series D preferred
Shares") of Preferred Stock, $0.01 par value (the "Series D Preferred Stock") of
the Company; and

WHEREAS, Equalnet and the Holder wish to amend the terms of the Series D
preferred Stock;

NOW, THEREFORE, in consideration of the premises and the representations,
warranties, covenants and agreements herein contained and intending to be
legally bound hereby, Equalnet and Holder hereby agree as follows:


                                  SECTION 1.
                     AMENDMENT OF STATEMENT OF RESOLUTIONS

a. Series D Preferred Stock.  The Company and the Holder agree that the
   ------------------------
   Statement of Resolutions of the Board of Directors Establishing and
   Designating Series D Convertible Preferred Stock and Fixing the Rights and
   Preferences of Such Series be amended to read as set forth in Annex I
                                                                 -------
   attached hereto (the "Amended Statement of Resolutions").

b. Approval of Amendments.  Holder shall vote in favor of the resolutions as set
   ----------------------
   forth in Annex IV attached hereto ratifying the amendments as set forth in
            --------
   Section 1.a at a Special Meeting of the Shareholders of Series D Preferred
   Stock to be held on or before Closing.

c. Closing.  The closing of this Agreement shall take place at the Law Offices
   -------
   of Brian W Pusch, Penthouse Suite, 29 West 57th Street, New York, New York
   10019 at 10:00 A.M., New York City Time, as soon as practicable, but no later
   than December 8, 1999, or at such other time and place as the Company and the
   Holder mutually agree upon orally or in writing (which time and place are
   designated as the "Closing").

d. Certain Terms.  The shares of Series D Preferred Stock issuable pursuant to
   -------------
   Section 5 of the Amended Statement of Resolutions as dividends on the Series
   D Preferred Shares are referred to herein as "Dividend Shares."  The shares
   of Common Stock issuable upon conversion of the Series D Preferred Shares and
   the Dividend Shares are referred to herein collectively as the "Common
   Shares."  The Series D Preferred Shares, the Dividend Shares and the Common
   Shares are referred to herein collectively as the "Securities."


                                  SECTION 2.
                  CONDITIONS PRECEDENT TO HOLDER'S OBLIGATION

The obligation of the Holder to ratify the amendment of the Series D Preferred
Stock as set forth
<PAGE>

herein is subject to the following conditions (any or all of which may be waived
by the Holder in its sole discretion):

a.  Representations and Warranties.  Each of the representations and warranties
    ------------------------------
    of the Company set forth in this Agreement shall be true on the date of the
    Closing.

b.  Documentation at Closing.  The Holder shall have received, on or prior to
    ------------------------
    the date of the Closing, all of the following, each in form and substance
    satisfactory to the Holder and its counsel:

    (1) A certificate, dated the date of the Closing, duly executed by the Chief
        Executive Officer certifying (1) the Articles of Incorporation and By-
        Laws of the Company as in effect on the date of the Closing; (2) all
        resolutions of the Board of Directors (and committees thereof) of the
        Company relating to this Agreement and the transactions contemplated
        hereby; and (3) such other matters as reasonably requested by the
        Holder;

    (2) A certificate, dated the date of the Closing, duly executed by the Chief
        Executive Officer certifying the statements of the Company to the effect
        set forth in Sections 2.a. and 2.c.;

    (3) Evidence of the filing with, and acceptance by, the Secretary of State
        of the State of Texas, of the Amended Statement of Resolutions;

    (4) An opinion of Dean H. Fisher, General Counsel of the Company, dated the
        date of the Closing, in form, scope and substance reasonably
        satisfactory to the Holder, to the effect set forth in Annex III
                                                               ---------
        attached hereto.

c.  Performance; No Default.  The Company shall have performed and complied with
    -----------------------
    all covenants and agreements contained in this Agreement required to be
    performed or complied with by the Company prior to or at the Closing.

d.  Proceedings and Documents. All corporate and other proceedings in connection
    -------------------------
    with the transactions contemplated by this Agreement and all documents and
    instruments incident to such transactions shall be satisfactory to the
    Holder and its counsel, and the Holder and its counsel shall have received
    all such counterpart originals or certified or other copies of such
    documents as they may reasonably request.


                                  SECTION 3.
                 CONDITIONS PRECEDENT TO EQUALNET'S OBLIGATION

The obligation of the Company to amend the Series D Preferred Stock as set forth
herein is subject to the following conditions (any or all of which may be waived
by the Company in its sole discretion):

a.  Representations and Warranties.  Each of the representations and warranties
    ------------------------------
    of the Holder set forth in this Agreement shall be true on the date of the
    Closing.

b.  Performance; No Default.  The Holder shall have performed and complied with
    -----------------------
    all agreements and conditions contained in this Agreement required to be
    performed or complied with by the Holder prior to or at the Closing.

                                       2
<PAGE>

c.  Proceedings and Documents. All corporate and other proceedings in connection
    -------------------------
    with the transactions contemplated by this Agreement and all documents and
    instruments incident to such transactions shall be satisfactory to the
    Company and its counsel, and the Company and its counsel shall have received
    all such counterpart originals or certified or other copies of such
    documents as they may reasonably request.


                                  SECTION 4.
           REPRESENTATIONS AND WARRANTIES AND AGREEMENTS OF EQUALNET

The Company represents and warrants to, and covenants and agrees with, the
Holder as follows:

a.  Organization and Standing of the Company. The Company is a corporation duly
    ----------------------------------------
    organized, validly existing and in good standing under the laws of its state
    of incorporation and has all requisite corporate power and authority (i) to
    own, lease and operate its properties, to carry on its business as now being
    conducted and (ii) to execute, deliver and perform its obligations under
    this Agreement, the Amended Statement of Resolutions, the Amended and
    Restated Registration Rights Agreement, the form of which is set forth in
    Annex II attached hereto (the "Amended and Restated Registration Rights
    --------
    Agreement"), and the other agreements to be executed by the Company in
    connection herewith and to consummate the transactions contemplated hereby
    and thereby. The Company and EqualNet Corporation, Netco Acquisitions Corp.
    or USC Telecom Corp. (the "Subsidiaries") are duly qualified to do business
    and are in good standing in all jurisdictions wherein such qualification is
    necessary and where failure so to qualify would have a material adverse
    effect on their business, properties, operations, condition (financial or
    other), results of operations or prospects of the Company. The Company has
    no equity investment in any person other than its subsidiaries.

b.  Concerning the Common Shares and the Common Stock. The Common Shares (1) to
    -------------------------------------------------
    the extent available out of the total shares currently authorized on the
    date hereof; and (2) subject to shareholder approval of the Reverse Stock
    Split as contemplated in Section 4.j herein, for such Common Shares
    unavailable out of the total shares authorized on the date hereof have been
    duly authorized and when issued upon conversion of the Preferred Shares or
    the Dividend Shares will be validly issued, fully paid and nonassessable and
    will not subject the holder thereof to personal liability by reason of being
    such holder. There are no preemptive or similar rights of any stockholder of
    the Company or any other person to acquire the Series D Preferred Shares and
    Dividend Shares or any of the Common Shares. The Common Stock is listed for
    trading on the Nasdaq SmallCap Market ("Nasdaq") and no suspension of
    trading in the Common Stock is in effect.

c.  Amendment Agreement, and Other Transaction Documents. This Agreement, the
    ----------------------------------------------------
    Amended and Restated Registration Rights Agreement, the Amended Statement of
    Resolutions (subject to Series D Shareholder approval of the Amended
    Statement of Resolutions) and the other agreements and instruments
    contemplated hereby and thereby have been duly and validly authorized by the
    Company, this Agreement has been duly executed and delivered by the Company
    and this Agreement is, and the Amended and Restated Registration Rights
    Agreement, the Amended Statement of Resolutions and such other agreements,
    when executed and delivered by the Company, will be, valid and binding
    obligations of the Company enforceable in accordance with their respective
    terms, subject as to enforceability to general principles of equity and to
    bankruptcy, insolvency, moratorium



                                       3
<PAGE>

    and other similar laws affecting the enforcement of creditor's rights
    generally.

d.  Non-contravention.  The execution and delivery by the Company of this
    -----------------
    Agreement and the other documents contemplated by this Agreement and the
    consummation by the Company of the issuance of the Series D Preferred Shares
    as contemplated by this Agreement, and the other transactions contemplated
    by this Agreement, the Amended and Restated Registration Rights Agreement
    and the Amended Statement of Resolutions do not and will not, with or
    without the giving of notice or the lapse of time, or both (i) result in any
    violation of any terms of the Articles of Incorporation or By-laws of the
    Company and the Subsidiaries; (ii) conflict with or result in a breach by
    the Company or the Subsidiaries or any of the terms or provisions of, or
    constitute a default under, or result in the modification, amendment,
    termination or cancellation of, result in the acceleration of any obligation
    of the Company or the Subsidiaries under, or result in the creation or
    imposition of any lien, security interest, charge or encumbrance upon any of
    the properties or assets of the Company or the Subsidiaries pursuant to any
    indenture, mortgage, deed of trust or other agreement or instrument to which
    the Company or the Subsidiaries is a party or by which the Company or the
    Subsidiaries or any of their properties or assets is bound or affected;
    (iii) violate or contravene any applicable law, rule or regulation or any
    applicable decree, judgment or order of any court, United States federal or
    state regulatory body, administrative agency or other governmental body
    having jurisdiction over the Company or the Subsidiaries or any of their
    properties or assets; or (iv) have any material adverse effect on any
    permit, certification, registration, approval, consent, license or franchise
    necessary for the Company or the Subsidiaries to own or lease and operate
    any of their properties or to conduct any of their businesses or the ability
    of the Company or the Subsidiaries to make use thereof.

e.  Approvals.  No authorization, approval or consent of, or filing with, any
    ---------
    court, governmental body, regulatory agency, self-regulatory organization,
    or stock exchange or market or the stockholders of the Company is required
    to be obtained or made by the Company for (1) the execution, delivery and
    performance by the Company of this Agreement, the Amended and Restated
    Registration Rights Agreement, and the other agreements and instruments
    contemplated hereby and thereby, (2) the execution and filing and
    performance by the Company of the Statement of Resolution, (3) the issuance
    of the Dividend Shares as contemplated by this Agreement and (4) the
    issuance of Common Shares on conversion of the Series D Preferred Shares or
    the issuance of Dividend Shares, other than (u) shareholder approval of the
    issue required by Section 5.h herein, (v) a reverse stock split
    substantially similar to that as described in Section 4.j herein; (w) the
    listing of the Common Shares on Nasdaq; (x) registration of the resale of
    the Common Shares under the 1933 Act as contemplated by the Amended and
    Restated Registration Rights Agreement and (y) Series D Shareholder approval
    of the Amended Statement of Resolutions.

f.  SEC Filings.  The Company has timely filed all required forms, reports and
    -----------
    other documents required to be filed with the Securities Exchange Commission
    (the "SEC") under the Securities Exchange Act of 1934, as amended (the "1934
    Act") since January 1, 1997. All of such forms, reports and other documents
    complied in all material respects with all applicable requirements of the
    1933 Act and the 1934 Act (1) when declared effective for those forms,
    reports and other documents filed under the 1933 Act; and (2) as filed or
    subsequently amended for those forms, reports and other documents filed
    under the 1934 Act.

g.  Absence of Brokers, Finders, Solicitation Compensation, Etc.  No broker,
    ------------------------------------------------------------
    finder or similar person is entitled to any commission, fee or other
    compensation in respect of the

                                       4
<PAGE>

    transactions contemplated by this Agreement by reason of any action or
    conduct of the Company or any person acting on its behalf, and the Company
    shall pay, and indemnify and hold harmless the Holder from, any claim made
    against the Holder by any person for any such commission, fee or other
    compensation. The Company has not and will not pay any commission or other
    remuneration to any person for solicitation of exchanges of Notes for shares
    of Series D Preferred Stock pursuant to this Agreement.

h.  [Reserved].

i.  [Reserved].

j.  Capitalization.  Following the reverse stock split as contemplated by the
    --------------
    Company (based on the number of shares outstanding on September 24, 1999),
    the authorized capital stock of the Company will be (a) 50,000,000 shares of
    Common Stock of which approximately 2,872,262 shares will be outstanding,
    all of which are fully paid and nonassessable; and (b) 5,000,000 shares of
    Preferred Stock, $.01 par value, of which 2,000 shares have been designated
    Series A Convertible Preferred Stock and of which 2,057 shares are
    outstanding, 3,000 shares have been designated Series B Senior Convertible
    Preferred Stock and of which 3,000 shares are outstanding, 300,000 shares
    have been designated Series C Convertible Preferred Stock and of which
    215,799 shares are outstanding, 6,500 shares have been designated Series D
    Convertible Preferred Stock and of which 3,964 shares are outstanding,
    900,000 shares have been designated Series E Convertible Preferred Stock
    ("Series E Preferred Stock") of which 833,333 are outstanding and of which
    6,500 shares will be designated as Series F Convertible Preferred Stock of
    which 3,142 shares are outstanding. As of September 24, 1999, the Company
    had outstanding options, warrants and similar rights, including preferred
    stock convertible into Common Stock, entitling the holders to purchase or
    acquire 25,324,269 shares of Common Stock. Other than as set forth in the
    preceding sentence, the Company does not have outstanding any material
    amount of securities (or obligation to issue any such securities)
    convertible into, exchangeable for or otherwise entitling the holders
    thereof to acquire shares of Common Stock. The Company has duly reserved
    from its authorized and unissued shares of Common Stock the full number of
    shares required for (a) all options, warrants, convertible securities and
    other rights to acquire shares of Common Stock which are outstanding and (b)
    all shares of Common Stock and options and other rights to acquire shares of
    Common Stock which may be issued or granted under the stock option and
    similar plans which have been adopted by the Company or any of its
    Subsidiaries other than those shares required for full conversion of Series
    A Convertible Preferred Stock, Series D Convertible Preferred Stock, Series
    E Junior Preferred Stock and Series F Convertible Preferred Stock.


                                  SECTION 5.
                               CERTAIN COVENANTS

a.  [Reserved].

b.  Nasdaq Listing; Reporting Status.  As soon as reasonably possible after the
    --------------------------------
    Closing Date and subject to the waiver in Section 5.e, the Company shall
    file an application for listing of additional shares with Nasdaq covering
    such of the Common Shares as shall not previously have been listed for
    trading on Nasdaq and shall provide evidence of such filing to the Holders.
    The Company shall use its best efforts to obtain the listing, subject to
    official notice of issuance, of such Common Shares and on Nasdaq. So long as
    the Holder beneficially

                                       5
<PAGE>

    owns any shares of Series F Preferred Stock or Common Shares, the Company
    shall timely file all reports required to be filed with the SEC pursuant to
    Section 13 or 15(d) of the 1934 Act, and the Company shall not terminate its
    status as an issuer required to file reports under the 1934 Act even if the
    1934 Act or the rules and regulations thereunder would permit such
    termination. So long as the Holder beneficially owns any shares of Series D
    Preferred Stock or Common Shares, the company shall timely file all reports
    required to be filed with the SEC pursuant to Section 13 or 15(d) of the
    1934 Act, and the Company shall not terminate its status as an issuer
    required to file reports under the 1934 Act even if the 1934 Act or the
    rules and regulations thereunder would permit such termination.

c.  Rule 144. The Company will use its best efforts to provide to the Holder
    --------
    and/or the Company's transfer agent an opinion of counsel, within 3 business
    days of receipt of a request for such opinion from the Holder or the
    transfer agent, to the effect that the shares of Common Stock issuable upon
    conversion of the Series D Preferred Stock may be resold without
    registration under the 1933 Act pursuant to Rule 144 thereof based upon
    customary representations by or on behalf of the Holder as such
    representations may be reasonably requested by the Company.

d.  Amended and Restated Registration Rights.  The parties hereto agree to enter
    ----------------------------------------
    into the Amended and Restated Registration Rights Agreement as of the
    Effective Date.

e.  Waivers. As of the Effective Date and until the earlier of (i) the Initial
    -------
    Reset Date as defined in the Amended Statement of Resolutions or (ii)
    January 15, 2000, the Holder waives (x) any requirement in the Amended
    Statement of Resolutions or in any related instrument (including Section 5.b
    herein) that the Company reserve a sufficient number of shares of Common
    Stock to permit conversion in full of the outstanding shares of Series D
    Preferred Stock or list such shares with Nasdaq; and (y) any rights Holder
    may have to demand redemption of the Series D Preferred Stock based on
    redemption events, inconvertibility events, repurchase events or events of
    default except those redemption rights that may arise from the Company's
    failure to timely perform its obligations to issue Common Stock upon
    conversion (subject to the limitations in Section 5.f) set forth in Section
    10 of the Statement of Resolutions; provided, however, that if the Closing
    Date does not occur on or before December 8, 1999, the waivers granted
    herein shall expire and cease to be effective.

f.  Agreed Limitation on Conversion.  The Holder agrees that, provided the
    -------------------------------
    Reverse Stock Split Date occurs on or before December 31, 1999, during the
    period commencing with the Reverse Stock Split Date and ending on the
    Trading Day immediately prior to the Initial Reset Date, as those terms are
    defined in the Amended Statement of Resolutions, Holder will not convert
    Series D Preferred Stock into Common Stock in an amount greater than the
    number of shares of Common Stock that the Holder would have had the ability
    to sell pursuant to Rule 144 during such period of time (the "Limitation
    Amount"). Holder also agrees that during the same time period of this
    Section 5.f, the redemption obligations of the Company upon inability to
    convert Series F Preferred Stock into Common Stock and related obligations
    to provide Inconvertibility Notices shall be limited to those instances
    created by the inability of the Company to issue the Limitation Amount of
    Common Stock upon conversion of Series D Preferred Stock.

g.  Call Option.  Until October 13, 2000, the Company shall have the option, but
    -----------
    not the obligation, to repurchase at any time, for a cash payment equal to
    $1,000 per share of Series D Preferred Stock so repurchased, any portion of
    Holder's Series D Preferred Stock that the Company in its sole discretion
    chooses to repurchase, upon 20 days prior written

                                       6
<PAGE>

    notice of exercise of this call option. This provision shall in no way be
    construed to limit Holder's conversion rights under the Amended Statement of
    Resolutions prior to such repurchase.

h.  Shareholder Approval.  The Company agrees to submit as soon as practicable,
    --------------------
    for purposes of Rule 4460 of the NASDAQ, a proposal for approval by
    shareholders of the Company's Common Stock allowing conversion of Series D
    Preferred Stock in amounts greater than the Maximum Share Amount, as such
    term is defined in the Amended Statement of Resolutions.

i.  Dividend Calculation. Holder agrees that for all dividends due from the
    --------------------
    Effective Date through Closing under the Amended Statement of Resolutions
    and not previously waived as to payment by any holder of Series D
    Convertible Preferred Stock, such dividends shall not be deemed untimely so
    long as they are paid on or before Closing.

                                  SECTION 6.
            REPRESENTATIONS AND WARRANTIES AND COVENANTS OF HOLDER

The Holder represents and warrants to, and covenants and agrees with, the
Company as follows:

a.  Amendment Agreement.  This Agreement has been duly and validly authorized,
    -------------------
    executed and delivered on behalf of the Holder and is a valid and binding
    agreement of the Holder enforceable in accordance with its terms, subject as
    to enforceability to general principles of equity and to bankruptcy,
    insolvency, moratorium and other similar laws affecting the enforcement of
    creditors' rights generally.


                                  SECTION 7.
                                 MISCELLANEOUS

a.  Costs, Expenses and Taxes.  Each party shall bear its own costs and expenses
    -------------------------
    in connection with the preparation, execution and delivery of this
    Agreement. The Company shall pay any and all stamp and other taxes payable
    or determined to be payable in connection with the execution and delivery of
    this Agreement.

b.  Survival of Representations.  The representations, warranties, covenants and
    ---------------------------
    agreements of the Holder and the Company contained in this Agreement or in
    any certificate furnished hereunder shall survive the Closing.

c.  Prior Agreements.  Except to the extent expressly amended hereby, the terms
    ----------------
    and provisions of the Purchase and Exchange Agreement is hereby confirmed
    and shall remain in full force and effect. Subject to the foregoing
    sentence, this Agreement constitutes the entire agreement between the
    parties concerning the subject matter hereof and supersedes any prior
    representations, understandings or agreements. There are no representations,
    warranties, agreements, conditions or covenants, of any nature whatsoever
    (whether express or implied, written or oral) between the parties hereto
    with respect to such subject matter except as expressly set forth herein.

d.  Severability.  The invalidity or unenforceability of any provision hereof
    ------------
    shall in no way affect the validity or enforceability of any other provision
    or the validity and enforceability of this Agreement in any other
    jurisdiction.

                                       7
<PAGE>

e.  GOVERNING LAW.  THIS AGREEMENT SHALL BE GOVERNED BY, AND CONSTRUED AND
    -------------
    ENFORCED IN ACCORDANCE WITH, THE LAWS OF THE STATE OF TEXAS, WITHOUT REGARD
    TO ITS CHOICE OF LAW RULES.

f.  Headings. Section headings in this Agreement are included herein for
    --------
    convenience of reference only and shall not constitute a part of, or affect
    the interpretation of, this Agreement.

g.  Counterparts.  This Agreement may be executed in any number of counterparts,
    ------------
    all of which taken together shall constitute one and the same instrument,
    and either of the parties hereto may execute this Agreement by signing any
    such counterpart. A facsimile transmission of this Agreement bearing a
    signature on behalf of a party hereto shall be legal and binding on such
    party.

h.  Binding Effect.  This Agreement shall be binding upon and inure to the
    --------------
    benefit of the parties hereto and their respective successors and permitted
    assigns.

i.  Further Assurances.  Each party to this Agreement will perform any and all
    ------------------
    acts and execute any and all documents as may be necessary and proper under
    the circumstances in order to accomplish the intents and purposes of this
    Agreement and to carry out its provisions.

j.  Publicity.   Neither the Company nor the Holder shall, nor shall they permit
    ---------
    their respective stockholders, directors, officers or advisors to, issue or
    cause the publication of any press release or make any other public
    statement, filing or announcement with respect to this Agreement and the
    transactions contemplated hereby without the prior approval of the other
    parties; provided, however, that the Company shall be entitled, without the
    prior approval of the Holder, to make any press release or other public
    disclosure with respect to such transactions as is required by applicable
    law or the Nasdaq Stock Market (although the Holder shall be consulted by
    the Company in connection with any such press release or other public
    disclosure prior to its release and shall be provided with a copy thereof).
    The Company and the Holder shall cooperate in issuing press releases or
    otherwise making public statements with respect to this Agreement and the
    transactions contemplated hereby, which cooperation shall include first
    consulting the other party hereto concerning the requirement for, and timing
    and content of, such public announcement.

[The rest of this page is intentionally left blank]

                                       8
<PAGE>

          IN WITNESS WHEREOF, the parties hereto, intending to be legally bound,
have caused this Agreement to be executed by their respective duly authorized
officers, as of the date first above written.


                                 EQUALNET COMMUNICATIONS CORP.



                                 By: /s/ Mitchell Bodian
                                     -------------------
                                     Name:  Mitchell Bodian
                                     Title: President & CEO


                                 WILLIS GROUP, LLC



                                 By: /s/ Mark Willis
                                     ---------------
                                     Name:  Mark Willis
                                     Title: President

                                       9