Document:

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                                                                   Exhibit 10(a)

                             THIRD AMENDMENT TO THE
                       UNITED STATES LIME & MINERALS, INC.

                          EMPLOYEE STOCK OWNERSHIP PLAN

WHEREAS, United States Lime & Minerals, Inc. (the "Corporation") amended and
restated the United States Lime & Minerals, Inc. Employee Stock Ownership
generally effective as of August 1, 1989 (the "Plan"); and

WHEREAS, the Corporation desires to amend the Plan in order to freeze
participation and benefits, merge the Plan with the United States Lime &
Minerals, Inc. 401(k) Profit Sharing Plan (the "401(k) Plan") and comply with
certain changes in the Internal Revenue Code; and

NOW, THEREFORE, pursuant to the powers reserved in Sections 10.01 and 10.03 of
the Plan, the Corporation does hereby amend the Plan effective as of July 31,
1999, unless another date is specified, as follows:

                                       I.

Section 1.01 of the Plan is amended by the addition of the following paragraph
thereto:

         "Notwithstanding anything to the contrary contained herein, in order to
         effectuate the Plan merger with the 401(k) Plan, participation and
         benefits shall be frozen as of the close of business on July 31, 1999.
         The Plan shall be merged with the 401(k) Plan as of the close of
         business on July 31, 1999 and assets shall be transferred as soon as
         administratively possible thereafter."

                                       II.

Effective August 1, 1999, Section 3.01 is hereby deleted in its entirety and the
following inserted in its stead:

         "Section 3.01  Eligibility Requirements.

         Except for Employees described in Section 3.02, all Employees shall be
         eligible to participate in the Plan. Notwithstanding the preceding
         sentence, no Employee shall be eligible to participate under the Plan
         on and after the close of business July 31, 1999."

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                                      III.

Effective August 1, 1998, Sections 4.07(c), 6.02, 6.03, 6.08 are amended by
deleting all references to $3,500 therein and replacing such references with
$5,000. The requirement that the benefit may not at any time have exceeded
$3,500 shall not apply to the $5,000 adjusted limit.

                                       IV.

Effective August 1, 1998, Section 6.08 of the Plan is amended by the addition of
the following language at the end of the first paragraph:

         "The commencement of benefits for a distribution may be less than 30
         days after the receipt of the written explanation of benefit and
         rollover options provided: (A) the Participant has been provided with
         information that clearly indicates that the Participant has at least 30
         days to consider the election; (B) the Participant is permitted to
         revoke any affirmative distribution election at least until the annuity
         starting date, or if later, at any time prior to the expiration of the
         7 day period that begins the day after the explanation is provided to
         the Participant; and (C) the annuity starting date is the date after
         the date that the written explanation was provided to the Participant."

                                 * * * * * * * *

IN WITNESS WHEREOF, the Corporation has caused this instrument to be executed as
of this _________ day of ____________________________, 1999.

                                           UNITED STATES LIME & MINERALS, INC.

                                           By:
                                              ----------------------------------

                                           Title:
                                                 -------------------------------

                                        2<PAGE>   1
                                                                   EXHIBIT 10.11

MORGAN STANLEY DEAN WITTER

                                                     1221 AVENUE OF THE AMERICAS
                                                     NEW YORK, NEW YORK 10020
                                                     (212) 761-4000

WebLink Wireless, Inc.
3333 Lee Parkway, Suite 100
Dallas, Texas 75219                                            January 27, 2000

Attention: John D. Beletic
            Chairman and Chief Executive Officer

Dear John:

We understand that WebLink Wireless, Inc. (the "COMPANY") intends to propose,
for approval by its stockholders at its 2000 regular annual meeting, the
Company's 2000 Flexible Incentive Plan substantially in the form approved by the
Board of Directors of the Company at its meeting on January 13, 2000 (the
"EMPLOYEE PLAN").  The Morgan Stanley stockholders support the adoption and
implementation of the Employee Plan in order to facilitate the retention and
motivation of the Company's key employees.

Accordingly, each of the undersigned stockholders of the Company hereby agrees,
severally and not jointly, to vote all shares of common stock, par value $.0001
per share, of the Company that such stockholder is entitled to vote at the time
of the 2000 regular annual meeting of the stockholders of the Company in favor
of the approval and adoption of the Employee Plan and any actions related
thereto.  Nothing herein shall be construed to limit in any way the ability of
the undersigned to sell, transfer or otherwise dispose of any such common stock
at any time and the agreement contained herein will be terminated with respect
to any such Shares that are sold, transferred or disposed of.

This letter agreement shall be governed by, and construed in accordance with,
the laws of the State of Delaware.

Very truly yours,

THE MORGAN STANLEY LEVERAGED EQUITY FUND II, L.P.
MORGAN STANLEY CAPITAL PARTNERS III, L.P.
MORGAN STANLEY CAPITAL INVESTORS, L.P.
MSCP III 892 INVESTORS, L.P.

By: /s/ Leigh J. Abramson
   --------------------------
   Leigh J. Abramson

MORGAN STANLEY VENTURE CAPITAL FUND II, L.P.
MORGAN STANLEY VENTURE CAPITAL FUND, L.P.
MORGAN STANLEY VENTURE CAPITAL FUND II, C.V.
MORGAN STANLEY VENTURE INVESTORS, L.P.

By: /s/ Guy de Chazal
   -----------------------
        Guy de Chazal<PAGE>   1
                                                                   EXHIBIT 10.13

                               AMENDMENT NO.1 TO
                          EQUIPMENT PURCHASE AGREEMENT

     Reference is hereby made to that Equipment Purchase Agreement (the
"Agreement") effective as of December 1, 1995 between PageMart, Inc.
("PageMart") and Motorola, Inc. ("Motorola"). Capitalized terms used but not
otherwise defined herein shall have the meanings ascribed thereto in the
Agreement.

     WHEREAS, PageMart and Motorola have agreed to modify the Agreement;

     NOW THEREFORE, the parties hereby agree as follows:

ARTICLE IV: Section 1.1. Buyer Purchase Commitment, is hereby amended to read
in its entirety, as follows:

1.1 Buyer Purchase Agreement. PageMart agrees to purchase from Motorola $40
million of conventional one-way and two-way infrastructure Equipment over the
forty-seven (47) month period beginning December 1, 1995 and ending October 31,
1999. In the event PageMart fails to order and take delivery from Motorola of
$40 million of infrastructure Equipment prior to September 1, 1999, provided any
failure to take delivery is not due to Force Majeure as described in Section
15.1 of Article I, or to Motorola's failure to meet reasonable scheduled
delivery dates, then on September 1, 1999, Motorola may prepare an invoice for,
and ship, infrastructure products chosen at Motorola's sole discretion, with a
total price (priced at the purchase prices of this Agreement) equivalent to the
difference between $40 million and the amount of infrastructure Equipment
previously ordered by PageMart and delivered under this Agreement. PageMart
agrees to accept such Products and to pay such invoice within thirty (30) days
of its receipt. The following orders shall apply towards PageMart's $40 million
commitment:

(A) PageMart shall issue to Motorola, a Purchase Order for Motorola's I-20 Nuc
IIs no later than June 19, 1998 for delivery to PageMart as per the attached
delivery schedule dated June 10, 1998.

(B) PageMart shall issue to Motorola, a Purchase Order for I-20 Kits no later
than June 19, 1998 for delivery to PageMart as per the attached delivery
schedule dated June 10, 1998. The total number of I-20 Kits to be ordered will
be based upon the number of Motorola transmitters in PageMart's network
requiring a kit.

(C) PageMart shall issue to Motorola, a Purchase Order for the RF-A! 1:5 (or a
same or similar product of equivalent functionality) no later than June 19, 1998
for delivery to PageMart as per the attached delivery schedule dated June 10,
1998. The obligation to place this order is subject to PageMart's
cost-effective, technical and organizational acceptance of this equipment
configuration.

(D) PageMart shall issue to Motorola, a Purchase Order for the RF-A! II no later
than October 31, 1998 for delivery to PageMart as per the attached delivery
schedule dated June 10, 1998. In the event Motorola, due to causes within its
control, unable to deliver the RF-A! II (with the capabilities specified in the
column entitled "RF-A! II" in the PageMart Receiver Requirements Document dated
June 10, 1998) by the delivery dates in the attached schedule, the October 31,
1999 date in this Section 1.1 shall be extended for the period of time that the
delivery is late, up to a maximum extension of one year, to October 31, 2000. If
the RF-A! II is not available, then at any time before April 1, 2000 if mutually
agreed by both parties or if after April 1, 2000, but before September 1, 2000,
PageMart may, in its sole discretion, cancel the Purchase Order for the RF-A! II
and issue a new Purchase Order for the remainder of the patent credit amount for
delivery on a schedule mutually agreed upon by PageMart and Motorola. The new
Purchase Order shall apply toward and fulfill PageMart's $40 million commitment,
even though the equipment may be delivered after September 1, 2000, but no later
than October 31, 2000.

This does not alter the credits provided for in the Catapult Agreement executed
12/30/97.

Date of Modification: June 15, 1998

PAGEMART WIRELESS, INC.            MOTOROLA, INC.

By: /s/ N. ROSS BUCKENHAM          By: /s/ TONY MARSHALL
   -----------------------------       ----------------------------

Name: N. Ross Buckenham            Name: Tony Marshall
     ---------------------------        ---------------------------

Title: President                   Title:  Vice President & General
                                             Manager APSD
      --------------------------         --------------------------

Date: 6/15/98                      Date:  June 15, 1998
     ---------------------------        ---------------------------

                                                     APPROVED AS TO
                                                     LEGALITY & FORM

                                                     15 JUNE 1998 TP
                                                     ---------------

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