<PAGE>

                                    GUARANTY
                                    --------

     GUARANTY, dated as of October 18, 2000, made by Michael V. Roberts, an
individual with an address at 1408 North Kingshighway, Suite 300, St. Louis, MO
63113 (the "GUARANTOR"), in favor of Alamosa Operations, Inc., a Delaware
corporation (the "LENDER").

                              W I T N E S S E T H :

     WHEREAS, Roberts Tower Company (the "BORROWER") and the Lender are parties
to a LOAN AGREEMENT dated as of October 18, 2000 (as amended from time to time,
the "LOAN AGREEMENT"); and

     WHEREAS, the Guarantor has ownership interests in the Borrower and derives
benefits from the Loan;

     NOW, THEREFORE, in consideration of the premises and the agreements herein,
the Guarantor hereby agrees with the Lender as follows:

     SECTION 1. Definitions. Reference is hereby made to the Loan Agreement for
a statement of the terms thereof. All terms used in this Guaranty which are
defined therein and not otherwise defined herein shall have the same meanings
herein as set forth therein.

     SECTION 2. Guaranty. The Guarantor hereby (a) irrevocably, absolutely and
unconditionally guarantees the prompt payment by the Borrower, as and when due
and payable (whether by scheduled maturity, required prepayment, acceleration,
demand or otherwise), of all amounts now or hereafter owing in respect of the
Note, the Loan Agreement and the other Loan Documents, including, without
limitation, the Loan, whether for principal, interest (including interest
accruing on or after the filing of any petition in bankruptcy or for
reorganization relating to the Borrower whether or not a claim for post-filing
interest is allowed in such proceeding), fees, expenses or otherwise (the
"GUARANTEED OBLIGATIONS"); and (b) agrees to pay any and all expenses (including
reasonable counsel fees and expenses) incurred by the Lender in enforcing its
rights under this Guaranty.

     SECTION 3. Guarantor's Obligations Unconditional.

     (a) The Guarantor hereby guarantees that the Guaranteed Obligations will be
paid strictly in accordance with the terms of the Loan Documents, regardless of
any law, regulation or order now or hereafter in effect in any jurisdiction
affecting any of such terms or the rights of the Lender with respect thereto.
The Guarantor agrees that his guarantee constitutes a guaranty of payment when
due and not of collection and waives any right to require that any resort be
made by the Lender to any other guaranty or any collateral that may secure any
of the Guaranteed Obligations. The obligations of the Guarantor under this
Guaranty are independent of the obligations under the Loan Agreement and the
other Loan Documents, and a separate action or actions may be brought and

<PAGE>

prosecuted against the Guarantor to enforce this Guaranty, irrespective of
whether any action is brought against the Borrower of any other guarantor if any
of the Guaranteed Obligations or whether the Borrower or any such other
guarantor is joined in any such action. The liability of the Guarantor hereunder
shall be absolute and unconditional irrespective of: (i) any lack of validity or
enforceability of any Loan Document or any agreement or instrument relating
thereto; (ii) any change in the time, manner or place of payment of, or in any
other term in respect of, all or any of the Guaranteed Obligations (including,
without limitation, any extension for longer than the original period), or any
other amendment or waiver of or consent to any departure from any provision of
any Loan Document other than this Guaranty (including the creation or existence
of any Guaranteed Obligations in excess of the amount permitted by any lending
formulas or other conditions contained in the Loan Documents or the amount
evidenced by the Loan Documents); (iii) any exchange or release of, or
non-perfection of any lien on or security interest in, any collateral, or any
release or amendment or waiver of or consent to any departure from any other
guaranty, for all or any of the Guaranteed Obligations; (iv) the existence of
any claim, set-off, defense or other right that the Guarantor may have against
any natural person, corporation, limited partnership, general partnership,
limited liability company, limited liability partnership, joint stock company,
joint venture, association, company, trust, bank, trust company, land trust,
business trust or other organizations, whether or not legal entities, and
Governmental Authorities (as hereinafter defined) (collectively, "Person"),
including, without limitation, the Lender, or (v) any other circumstance which
might otherwise constitute a defense available to, or a discharge of, the
Borrower or any other guarantor in respect of the Guaranteed Obligations or the
Guarantor in respect hereof.

     (b) This Guaranty (i) is a continuing guaranty and shall remain in full
force and effect until such date on which all of the Guaranteed Obligations and
all other expenses to be paid by the Guarantor pursuant hereto shall have been
indefeasibly paid in full in cash and (ii) shall continue to be effective or
shall be reinstated, as the case may be, if at any time any payment of any of
the Guaranteed Obligations is rescinded or must otherwise be returned by the
Lender upon the insolvency, bankruptcy or reorganization of the Borrower or
otherwise, all as though such payment had not been made.

     SECTION 4. Waivers. The Guarantor hereby waives, to the extent permitted by
applicable law, (i) promptness and diligence; (ii) notice of acceptance and
notice of the incurrence of any Guaranteed Obligation by the Borrower; (iii)
notice of any actions taken by the Lender under any Loan Document or any other
agreement or instrument relating thereto; (iv) all other notices, demands and
protests, and all other formalities of every kind in connection with the
enforcement of the Guaranteed Obligations or of the obligations of the Guarantor
hereunder, the omission of or delay in which, but for the provisions of this
Section 4, might constitute grounds for relieving the Guarantor of his
obligations hereunder; (v) any right to compel or direct the Lender to seek
payment or recovery of any amounts owed under this Guaranty from any one
particular fund or source; (vi) any requirement that the Lender protect, secure,
perfect or insure any Lien (as hereinafter defined) or any property subject
thereto or exhaust any right or take any action against the Borrower or any
other Person and (vii) any other defense available to the Guarantor.

     SECTION 5. Subrogation. Until the indefeasible payment in full in cash of
all Guaranteed Obligations together with all other expenses to be paid by the
Guarantor and the termination of the Loan Agreement, the Guarantor hereby
irrevocably waives and agrees he will not exercise any and all rights which he
has or may have at any time or from time to time (whether arising directly or
indirectly by operation of law or contract) to assert any claim against the
Borrower on account of any payments made under this Agreement, including,
without limitation, any and all

                                       2
<PAGE>

existing and future rights of subrogation, reimbursement, exoneration,
contribution and/or indemnity. If any amount shall be paid to the Guarantor on
account of such subrogation rights at any time when all of the Guaranteed
Obligations and all such other expenses shall not have been paid in full, such
amount shall be (a) held in trust for the benefit of the Lender, (b) identified
on the books and records of Guarantor as the Lender's property, and (c)
segregated from the other funds of the Guarantor and shall forthwith be paid
over to the Lender to be applied in whole or in part by the Lender against the
Guaranteed Obligations, whether matured or unmatured, and all such other
expenses in accordance with the terms of the Loan Agreement.

     SECTION 6. Representations and Warranties. The Guarantor hereby represents
and warrants as follows:

     (a) The Guarantor has the legal capacity to own his assets, execute,
deliver and perform this Guaranty.

     (b) The execution, delivery and performance by the Guarantor of this
Guaranty (i) does not and will not contravene any applicable law; (ii) does not
and will not violate any contractual restriction binding on or otherwise
affecting the Guarantor or any of his assets or properties; and (iii) does not
and will not result in or require the creation of any (x) lien, mortgage,
pledge, assignment, security interest, charge or encumbrance of any kind
(including any agreement to give any of the foregoing, any conditional sale or
other title retention agreement and any lease in the nature thereof) and any
option, trust or other preferential arrangement having the practical effect of
any of the foregoing and (y) in the case of securities, any purchase option,
call or similar right of a third party with respect to such securities ("LIEN")
upon or with respect to any of the Guarantor's properties.

     (c) No authorization, approval or other action by, and no notice to or
filing with, any federal, state, provincial, municipal, national or other
government, governmental department, commission, board, bureau, court, agency or
instrumentality or political subdivision thereof or any entity or officer
exercising executive, legislative, judicial, regulatory or administrative
functions of or pertaining to any government or any court, in each case whether
associated with a state of the United States, the United States, or a foreign
entity or government ("GOVERNMENTAL AUTHORITY") or other regulatory body is
required in connection with the due execution, delivery and performance by the
Guarantor of this Guaranty.

     (d) This Guaranty is a legal, valid and binding obligation of the
Guarantor, enforceable against the Guarantor in accordance with its terms,
except as may be limited by applicable bankruptcy, insolvency, reorganization,
moratorium or other similar laws and general principles affecting the
enforcement of creditor's rights generally.

     (e) There is no pending or, to the best knowledge of the Guarantor after
due inquiry, threatened action, suit or proceeding against the Guarantor or to
which any of the assets of the Guarantor is subject, before any court or other
Governmental Authority or any arbitrator (i) which challenges the validity or
enforceability of this Guaranty, or (ii) in which there is a reasonable
possibility of an adverse decision which would materially adversely affect the
condition (financial or otherwise) of the Guarantor.

     (f) The Guarantor now has and will continue to have independent means of
obtaining information concerning the affairs and financial condition of the
Borrower, and has no

                                       3
<PAGE>

need of, or right to obtain from the Lender, any credit or other information
concerning the affairs or financial condition of the Borrower that may come
under the control of the Lender.

     (g) All federal, state, local and foreign tax returns, reports and
statements (collectively, the "TAX RETURNS") required to be filed by the
Guarantor have been filed with the appropriate governmental agencies in all
jurisdictions in which such Tax Returns are required to be filed (after giving
effect to any extensions obtained for the filing thereof), all such Tax Returns
are true and correct in all material respects, and all taxes, charges and other
impositions due and payable have been timely paid prior to the date on which any
fine, penalty, interest, late charge or loss may be added thereto for
non-payment thereof.

     (h) No written statement prepared or furnished to the Lender in connection
with the transactions contemplated hereby, by or on behalf of the Guarantor,
when all such statements are taken as a whole, contains any untrue statement of
a material fact or omits to state a material fact necessary to make the
statements contained therein not misleading. All facts known to the Guarantor
which are material to an understanding of the financial condition, business,
properties or prospects of the Guarantor and the Borrower have been disclosed to
the Lender.

     (i) The Guarantor is not in default under or with respect to any
contractual obligation owed by the Guarantor and, to the knowledge of the
Guarantor after due inquiry, no other party is in default under or with respect
to any contractual obligation owed to the Guarantor.

     (j) No Event of Default (as hereinafter defined) or event which, with the
lapse of time and/or notice, would become an Event of Default (such event, a
"DEFAULT") has occurred and is continuing.

     SECTION 7. Covenants. Until the indefeasible payment in full in cash of all
Guaranteed Obligations and all other expenses to be paid by the Guarantor:

     (a) The Guarantor shall furnish to the Lender no later than December 1,
2000 a summary of assets and liabilities of the Guarantor as of June 30, 2000,
in form and substance acceptable to the Lender, together with a schedule of
contingent liabilities, signed by the Guarantor as fairly presenting his
financial condition as of such date.

     (b) The Guarantor shall have a minimum net worth of $ 25 million.

     (c) The Guarantor shall comply in all material respects with all
requirements of law, contractual obligations, commitments, instruments,
licenses, permits and franchises, the noncompliance with could reasonably be
expected to have a material adverse effect on (i) the business, assets,
revenues, results of operations, or financial condition of the Borrower, (ii)
the ability of the Borrower to perform its material obligations under the Loan
Documents or (iv) the validity or enforceability of the Loan Documents, the
Lender's Liens or the Lender's rights and remedies under the Loan Documents
("MATERIAL ADVERSE EFFECT").

     (d) The Guarantor shall pay and discharge, before the same shall become
delinquent, all lawful governmental claims, taxes, assessments, charges and
levies.

     (e) The Guarantor shall furnish to the Lender promptly and in any event
within two business days after the Guarantor becomes (or should have become)
aware of the existence of

                                       4
<PAGE>

(i) any breach or non-performance of, or any default under, any contractual
obligation which is material to the business, prospects operations or financial
condition of the Borrower or the Guarantor, as the case may be, or (ii) any
event, development or other circumstances which has any reasonable likelihood of
causing of resulting in a Material Adverse Effect, written notice in reasonable
detail specifying the nature of the breach, non-performance, default, event,
development or circumstance, including without limitation, the anticipated
effect thereof, which notice shall be promptly confirmed in writing within five
days.

     (f) (A) If the Mergers are not consummated and the Guaranteed Obligations
are not paid in full , the Guarantor shall, within ninety (90) days after the
date of a written notice delivered by Lender to RWC advising RWC that the Merger
Agreement is being terminated and that the Mergers will not be consummated,
assign, transfer and convey to Lender, without recourse, all or a portion of his
membership interests in RWC so that Lender shall own, in addition to any other
membership interests in RWC that the Lender may already own or have the right to
own under any other agreement with RWC, the Guarantor or others, on a fully
diluted basis, 10% of the total memberships interests in RWC, free and clear of
all liens and encumbrances. Such obligation of the Guarantor is evidenced by the
Assignment Agreement in the Form of Exhibition B to the Loan Agreement.

         (B) If (x) the Mergers are consummated, and (y) the Guaranteed
Obligations are not paid in full prior to or simultaneously with the Parent
Merger, the Roberts agree that Lender shall be entitled to retain all or a
portion of the cash and stock due to the Roberts as part of the Parent Merger
consideration and apply such consideration towards the payment of the Guaranteed
Obligations.

     SECTION 8. Events of Default. Each of the following shall constitute an
"EVENT OF DEFAULT":

     (a) The Guarantor shall fail to perform any term, covenant or agreement
   contained herein;

     (b) Any representation or warranty of the Guarantor herein shall prove to
   have been false in any material respect as of the date on which it was made
   or deemed to have been made;

     (c) The Guarantor shall die or become incapacitated;

     (d) The Guarantor shall (i) apply for or consent to the appointment of a
   receiver, custodian, trustee or liquidator; (ii) be generally unable to pay
   his debts as such debts become due; (iii) make a general assignment for the
   benefit of the Guarantor's creditors; (iv) commence a voluntary case under
   the United States Bankruptcy Code (as now or hereafter in effect); (v) file a
   petition seeking to take advantage of any other law of any jurisdiction
   relating to bankruptcy, insolvency, or composition or readjustment of debts;
   (vi) fail to controvert in a timely and appropriate manner, or acquiesce in
   writing to, any petition filed against the Guarantor in an involuntary case
   under the United States Bankruptcy Code; or (vii) take any action for the
   purpose of effecting any of the foregoing;

     (e) A proceeding or case shall be commenced, without the application or
   consent of the Guarantor, in any court of competent jurisdiction, seeking (i)
   the liquidation of his assets,

                                       5
<PAGE>

   or the composition or readjustment of his debts, (ii) the appointment of a
   trustee, receiver, custodian, liquidator or the like of any substantial part
   of his assets, or (iii) similar relief in respect of the Guarantor under any
   law of any jurisdiction relating to bankruptcy, insolvency, or the
   composition or readjustment of debts, and such proceedings or case shall
   continue undismissed, or an order, judgment or decree approving or ordering
   any of the foregoing shall be entered and continue unstayed and in effect for
   a period of sixty (60) days; or an order for relief against the Guarantor
   shall be entered in an involuntary case under any bankruptcy, insolvency,
   composition, readjustment of debt, liquidation of assets or similar law of
   any jurisdiction; or

     (f) The Guarantor shall disavow this Guaranty.

     SECTION 9. Notices, Etc. All notices and other communications provided for
hereunder shall be in writing and shall be mailed, telecopied or delivered, if
to the Guarantor, to the address set forth below his signature to this Guaranty;
if to the Lender, to it at its address set forth in the Loan Agreement; or, as
to any such Person, at such other address as shall be designated by said Person
in a written notice to such other Person complying as to delivery with the terms
of this Section 9. All such notices and other communications shall be effective
(i) if mailed, when received or three business days after mailing, whichever
first occurs, (ii) if telecopied, when transmitted and a confirmation is
received, or (iii) if delivered, upon delivery.

     SECTION 10. Consent to Jurisdiction; Waiver of Immunities.

     (a) The Guarantor hereby irrevocably submits to the jurisdiction of any
Texas State or Federal court sitting in _________ in any action or proceeding
arising out of or relating to this Guaranty, and the Guarantor hereby
irrevocably agrees that all claims in respect of such action or proceeding may
be heard and determined in such Texas State or Federal court. The Guarantor
irrevocably consents to the service of any and all process in any such action or
proceeding by the mailing (by certified or registered mail, postage prepaid and
return receipt requested) or delivering a copy of such process to the Guarantor
at its address specified in Section 9 hereof, in the case of mailing, such
service to become effective thirty (30) days after such mailing. The Guarantor
agrees that a final judgment in any such action or proceeding shall be
conclusive and may be enforced in other jurisdictions by suit on the judgment or
in any other manner provided by law.

     (b) Nothing in this Section 10 shall affect the right of the Lender to
serve legal process in any other manner permitted by law or affect the right of
the Lender to bring any action or proceeding against the Guarantor or its
property in the courts of any other jurisdictions.

     (c) The Guarantor hereby expressly and irrevocably waives, to the fullest
extent permitted by law, any objection which he may now or hereafter have to the
laying of venue of any such action or proceeding brought in any such court
referred to above and any claim that any such action or proceeding has been
brought in an inconvenient forum. To the extent that the Guarantor has or
hereafter may acquire any immunity from jurisdiction of any court or from any
legal process (whether through service or notice, attachment prior to judgment,
attachment in aid of execution or otherwise) with respect to himself or his
property, the Guarantor hereby irrevocably waives such immunity in respect of
his obligations under this Guaranty and the other Loan Documents.

     SECTION 11. WAIVER OF JURY TRIAL. EACH OF THE GUARANTOR AND, BY ACCEPTANCE
HEREOF, THE LENDER WAIVES ANY RIGHT TO A TRIAL BY JURY

                                       6
<PAGE>

IN ANY ACTION, PROCEEDING OR COUNTERCLAIM CONCERNING ANY RIGHTS UNDER THIS
GUARANTY, THE LOAN DOCUMENTS OR UNDER ANY AMENDMENT, WAIVER, CONSENT,
INSTRUMENT, DOCUMENT OR OTHER AGREEMENT DELIVERED OR WHICH IN THE FUTURE MAY BE
DELIVERED IN CONNECTION THEREWITH, OR ARISING FROM ANY RELATIONSHIP EXISTING IN
CONNECTION WITH THIS GUARANTY, AND AGREES THAT ANY SUCH ACTION, PROCEEDING OR
COUNTERCLAIM SHALL BE TRIED BEFORE A COURT AND NOT BEFORE A JURY.

     SECTION 12. Miscellaneous.

     (a) The Guarantor will make each payment hereunder in lawful money of the
United States of America and in immediately available funds to the Lender, at
such address specified by the Lender from time to time by notice to the
Guarantor.

     (b) No amendment of any provision of this Guaranty shall be effective
unless it is in writing and signed by the Guarantor and the Lender, and no
waiver of any provision of this Guaranty, and no consent to any departure by the
Guarantor therefrom, shall be effective unless it is in writing and signed by
the Lender, and then such waiver or consent shall be effective only in the
specific instance and for the specific purpose for which given.

     (c) No failure on the part of the Lender, to exercise, and no delay in
exercising, any right hereunder or under any other Loan Document shall operate
as a waiver thereof, nor shall any single or partial exercise of any right
preclude any other or further exercise thereof or the exercise of any other
right. The rights and remedies of the Lender provided herein and in the other
Loan Documents are cumulative and are in addition to, and not exclusive of, any
rights or remedies provided by law. The rights of the Lender under any Loan
Document against any party thereto are not conditional or contingent on any
attempt by the Lender to exercise any of its rights under any other Loan
Document against such party or against any other Person.

     (d) Any provision of this Guaranty which is prohibited or unenforceable in
any jurisdiction shall, as to such jurisdiction, be ineffective to the extent of
such prohibition or unenforceability without invalidating the remaining portions
hereof or thereof or affecting the validity or enforceability of such provision
in any other jurisdiction.

     (e) This Guaranty shall (i) be binding on the Guarantor and his heirs and
legal representatives, and (ii) inure, together with all rights and remedies of
the Lender hereunder, to the benefit of the Lender and its successors or
transferees in respect of the Note and the other Loan Documents.

     (f) At any time and from time to time, upon the request of the Lender, the
Guarantor shall execute, deliver and acknowledge, or cause to be executed,
delivered and acknowledged, such further documents and instruments and do such
other acts and things as the Lender may reasonably request in order to effect
fully the intent and purposes of this Guaranty and the Loan Documents, and any
other agreements, instruments and documents delivered pursuant hereto or in
connection with the making of the Loan, in proper form for recording and
otherwise in form and substance reasonably satisfactory to the Lender and its
counsel.

                                       7
<PAGE>

     (g) This Guaranty shall be governed by and construed and enforced in
accordance with the internal laws of the State of Texas applicable to contracts
made and to be performed therein without regard to conflict of law principles.



                  [REMAINDER OF PAGE INTENTIONALLY LEFT BLANK]
















                                       8
<PAGE>

     IN WITNESS WHEREOF, the Guarantor has caused this Guaranty to be executed
as of the date first above written.

                                         By:/s/ Michael V. Roberts
                                            ------------------------------------
                                            Name: Michael V. Roberts
                                            Address:
                                                     ----------
                                            1408 N. Kingshighway
                                            --------------------
                                            St. Louis, MO 63113
                                            --------------------

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