<PAGE>   1
                                                                 EXHIBIT 10.15


                       FIRST AMENDMENT TO CREDIT AGREEMENT

         THIS FIRST AMENDMENT TO CREDIT AGREEMENT ("Amendment") is made and
entered into as of August 21, 2000 (the "Amendment Date"), by and among XETA
Technologies, Inc., an Oklahoma corporation, formerly known as Xeta Corporation
(the "Borrower"), Bank One, Oklahoma NA, and First Star Bank, N.A., formerly
known as Mercantile Bank, N.A., as Lenders under the Credit Agreement referred
to below (the "Lenders"), and Bank One, Oklahoma, NA, as Agent (in such
capacity, the "Agent"), with reference to the following:

         A. The Borrower, the Lenders, the Agent and Banc One Capital Markets,
Inc., as Lead Arranger and Sole Book Runner, are parties to that certain Credit
Agreement dated as of November 30, 1999 (the "Credit Agreement"), pursuant to
which the Lenders severally agreed to make Loans to the Borrower under three (3)
Credit Facilities therein described.

         B. The Borrower has requested that the Lenders increase the Aggregate
Revolving Commitment (and correspondingly, the amount available under the
Revolving Credit Facility) from $5,000,000 to $8,000,000.

         C. The Lenders have agreed to the requested increase in the Aggregate
Revolving Credit Commitment, but only upon the terms and conditions set forth in
this Amendment.

         NOW, THEREFORE, in consideration of the premises and the mutual
covenants and agreements herein contained, and for other good and valuable
consideration, the receipt and adequacy of which are hereby acknowledged, the
parties hereby amend the Credit Agreement, effective as of the Amendment Date,
as follows:

1. DEFINITIONS. Capitalized terms used herein (including capitalized terms used
in the recitals above) but not otherwise defined have the respective meanings
assigned to them in the Credit Agreement.

2. INCREASE IN REVOLVING CREDIT COMMITMENTS. The Revolving Credit Commitment of
Bank One, Oklahoma, NA is hereby increased from $2,750,000 to $4,400,000, and
the Revolving Credit Commitment of Firstar Bank, N.A. is hereby increased
from $2,250,000 to $3,600,000. As a result of the foregoing increases, the
parties acknowledge and agree (i) that the Aggregate Revolving Credit Commitment
is increased from $5,000,000 to $8,000,000, (ii) the term "Obligations" as used
in the Credit Agreement and any other Loan Documents will include all Loans from
time to time outstanding under the Revolving Credit Facility (as increased
hereby), and (ii) from and after the Amendment Date, the commitment fee payable
under Section 2.5 of the Credit Agreement will be based on the daily unused
portion of each Lender's Revolving Credit Commitment (as increased hereby) and
Acquisition Loan Commitment. In order to evidence the foregoing increases, the
Borrower agrees to make, execute and deliver to each Lender a replacement
Revolving Note (collectively, the "Replacement Revolving Notes"), substantially
in the form attached hereto as Exhibit "E-1A" (with appropriate insertions), in
increase and replacement of and substitution for the Revolving Notes delivered
at the Closing.


<PAGE>   2


3. CHANGE OF CORPORATE NAME. The parties acknowledge that the corporate name of
the Borrower was changed effective April 11, 2000, to XETA Technologies, Inc.
The Borrower agrees to execute such amendments to financing statement or new
financing statements as may be necessary or advisable to reflect such name
change and to reimburse the Agent for all costs and expenses, including
reasonable attorney's fees and filing fees, that the Agent may incur in causing
the same to be prepared, executed and filed of record.

4. CONDITIONS PRECEDENT. This Amendment shall become effective as of the
Amendment Date, subject to the Borrower's satisfaction of the following
conditions precedent (in addition to the conditions precedent set forth in
Article IV of the Credit Agreement):

         A. Execution of Documents. This Amendment and the Replacement Revolving
Notes shall have been duly and validly authorized, executed and delivered to the
Agent and the Lenders by the Borrower.

         B. Resolutions. The Bank shall have received a copy of the resolutions
of the Board of Directors of the Borrower authorizing the increase in the
Revolving Credit Facility and the execution, delivery and performance of this
Amendment, the Credit Agreement (as amended by this Amendment) and the
Replacement Revolving Notes.

         C. Accuracy of Representations and Warranties. All representations and
warranties made by the Borrower in the Credit Agreement and the other Loan
Documents and in Section 5 hereof shall be true and correct in all material
respects as of the Amendment Date (except to the extent any of such
representations and warranties with respect to the financial condition of the
Borrower refer to an earlier specified date).

         D. No Default. There shall not have occurred any Default or Unmatured
Default as of the Amendment Date, and the Borrower shall be current in payment
of all principal, interest and fees due and owing to the Agent or the Lenders as
of the Amendment Date.

         E. Commitment Fee. The Borrower shall have paid to the Agent for the
account of each Lender, in accordance with each Lender's Pro Rata Share, a
commitment fee of $6,000 (which amount is 0.20% of the increase in the
Aggregate Revolving Credit Commitment).

5. REPRESENTATIONS AND WARRANTIES. All representations and warranties of
Borrower contained in Article V of the Credit Agreement are hereby remade and
restated as the date hereof and shall survive the execution and delivery of this
Amendment. The Borrower further represents and warrants as follows:

         A. Authority. The Borrower has all requisite power and authority and
has been duly authorized to increase the amount of the Aggregate Revolving
Credit Commitment, to borrow under the Revolving Credit Facility (as increased
hereby), and to execute, deliver and perform its obligations under this
Amendment, the Credit Agreement (as amended by this Amendment) and the
Replacement Revolving Notes.


<PAGE>   3


         B. Binding Obligations; Enforceability. This Amendment, the Credit
Agreement (as amended by this Amendment) and the Replacement Revolving Notes are
valid and legally binding obligations of the Borrower, enforceable in accordance
with their respective terms, except as limited by applicable bankruptcy,
insolvency or other laws affecting the enforcement of creditors' rights
generally.

         C. No Conflict; Government Consent. Neither the execution and delivery
by the Borrower of this Amendment and the Replacement Revolving Notes, nor the
consummation of the transactions therein contemplated, nor compliance with the
provisions thereof, will violate (i) any law, rule, regulation, order, writ,
judgment, injunction, decree or award binding on the Borrower or any of its
Subsidiaries, or (ii) the Borrower's or any Subsidiary's articles or certificate
of incorporation, partnership agreement, certificate of partnership, articles or
certificate of organization, by-laws, or operating or other management
agreement, as the case may be, or (iii) the provisions of any indenture,
instrument or agreement to which the Borrower or any of its Subsidiaries is a
party or is subject, or by which it, or its Property, is bound, or conflict with
or constitute a default thereunder, or result in, or require, the creation or
imposition of any Lien in, of or on the Property of the Borrower or any of its
Subsidiaries pursuant to the terms of any such indenture, instrument or
agreement. No order, consent, adjudication, approval, license, authorization, or
validation of, or filing, recording or registration with, or exemption by, or
other action in respect of any governmental or public body or authority, or any
subdivision thereof, which has not been obtained by the Borrower or any of its
Subsidiaries, is required to be obtained by the Borrower or any of its
Subsidiaries in connection with the execution and delivery of this Amendment or
the Replacement Revolving Note, the borrowings under the Credit Agreement (as
amended hereby), the payment and performance by the Borrower of the Obligations,
or the legality, validity, binding effect or enforceability of this Amendment,
the Credit Agreement (as amended by this Amendment) or the Replacement Revolving
Notes.

         D. No Material Adverse Change. Since April 30, 2000 (the date of the
latest financial statements of the Borrower which have been delivered to the
Agent and the Lenders), there has been no adverse change in the business,
Property, prospects, condition (financial or otherwise) or results of operations
of the Borrower and its Subsidiaries which could reasonably be expected to have
a Material Adverse Effect.

6. MISCELLANEOUS.

         A. Effect of Amendment. The terms of this Amendment shall be
incorporated into and form a part of the Credit Agreement. Except as amended,
modified and supplemented by this Amendment, the Credit Agreement shall continue
in full force and effect in accordance with its original stated terms, all of
which are hereby reaffirmed in every respect as of the date hereof. In the event
of any irreconcilable inconsistency between the terms of this Amendment and the
terms of the Credit Agreement, the terms of this Amendment shall control and
govern, and the agreements shall be interpreted so as to carry out and give full
effect to the intent of this Amendment. All references to the "Credit Agreement"
appearing in any of the Loan Documents shall hereafter be deemed references to
the Credit Agreement as amended, modified and supplemented by this Amendment,
and all references to the "Revolving Notes" appearing in the



<PAGE>   4

Credit Agreement and any of the other Loan Documents shall hereafter be deemed
references to the Replacement Revolving Notes.

         B. Exhibits. The form of Replacement Revolving Note attached hereto as
Exhibit E-1A is hereby substituted for Exhibit E-1 to the Credit Agreement and
incorporated therein by this reference.

         C. Descriptive Headings. The descriptive headings of the several
sections of this Amendment are inserted for convenience only and shall not be
used in the construction of the content of this Amendment.

         D. Governing Law. This Amendment shall be construed and enforced in
accordance with, and the rights of the parties shall be governed by, the laws of
the State of Oklahoma.

         E. Reimbursement of Expenses. The Borrower agrees to pay the reasonable
fees and out-of-pocket expenses of Crowe & Dunlevy, counsel to the Agent,
incurred in connection with the preparation of this Amendment and the
consummation of the transactions contemplated hereby.

         IN WITNESS WHEREOF, the Borrower, the Lenders and the Agent have
executed this Amendment as of the Amendment Date.

                                       XETA TECHNOLOGIES, INC.,
                                       formerly known as Xeta Corporation


                                       By: /s/ ROBERT B. WAGNER
                                          -------------------------------------
                                       Name:   Robert B. Wagner
                                            -----------------------------------
                                       Title: VP Finance
                                             -----------------------------------


                                       BANK ONE, OKLAHOMA, NA,
                                       Individually and as Agent

                                       By: /s/ TIMOTHY T. KOSKI
                                          -------------------------------------
                                       Name: Timothy T. Koski
                                            -----------------------------------
                                       Title: Vice President
                                             -----------------------------------


                                       FIRST STAR BANK, N.A., formerly known as
                                       Mercantile Bank, N.A.


                                       By: /s/ GREG VATTEROTT
                                          -------------------------------------
                                       Name:   Greg Vatterott
                                            -----------------------------------
                                       Title:  Vice President
                                             -----------------------------------

<PAGE>   5

                              CONSENT OF GUARANTOR

         The undersigned hereby (i) acknowledges and consents to the execution
and delivery of the above and foregoing First Amendment to Credit Agreement,
(ii) confirms that the Subsidiary Guaranty of the undersigned will continue in
full force and effect as security for payment and performance of all of the
"Guaranteed Obligations," as such term is used in the Subsidiary Guaranty,
including all Loans from time to time outstanding under the Revolving Credit
Facility (as increased by the above and foregoing First Amendment to Credit
Agreement), and (iii) ratifies and reaffirms the Subsidiary Guaranty.

         No inference shall be drawn from the undersigned's execution of this
Consent that consent or approval of the undersigned is required for this or any
future modification or amendment of or supplement to the Credit Agreement or
other Loan Document, or for this or any future increase, extension or renewal of
the Guaranteed Obligations.

         Capitalized terms used in this Consent and not otherwise defined have
the respective meanings assigned to them in the Credit Agreement referred to in
the above and foregoing First Amendment to Credit Agreement.


                                       U.S. TECHNOLOGIES SYSTEMS, INC.


                                       By: /s/ ROBERT B. WAGNER
                                          -------------------------------------
                                       Name:   Robert B. Wagner
                                            -----------------------------------
                                       Title:  VP Finance CFO
                                             -----------------------------------



<PAGE>   6




                                 REVOLVING NOTE

$4,400,000                                                     August 21, 2000


         XETA Technologies, Inc., an Oklahoma corporation, formerly known as
Xeta Corporation (the "Borrower"), promises to pay to the order of Bank One,
Oklahoma, NA (the "Lender"), the aggregate unpaid principal amount of all
Revolving Loans made by the Lender to the Borrower pursuant to Section 2.1.1 of
the Agreement (as hereinafter defined), in immediately available funds at the
main office of Bank One, Oklahoma, NA, Tulsa, Oklahoma, as Agent, together with
interest on the unpaid principal amount hereof at the rates and on the dates set
forth in the Agreement. The Borrower shall pay the principal of and accrued and
unpaid interest on the Revolving Loans in full on the Revolving Credit Facility
Termination Date and shall make such mandatory payments as are required to be
made under the terms of Article II of the Agreement.

         The Lender shall, and is hereby authorized to, record on the schedule
attached hereto, or to otherwise record in accordance with its usual practice,
the date and amount of each Revolving Loan and the date and amount of each
principal payment hereunder.

         This Note is one of the Revolving Notes issued pursuant to, and is
entitled to the benefits of, the Credit Agreement dated as of November 30, 1999,
as amended by the First Amendment to Credit Agreement dated as of August 21,
2000 (which, as it may be further amended or modified and in effect from time to
time, is herein called the "Agreement"), among the Borrower, the lenders party
thereto, including the Lender, and Bank One, Oklahoma, NA, as Agent. Capitalized
terms used herein and not otherwise defined herein are used with the meanings
attributed to them in the Agreement. Reference is made to the Agreement for a
statement of the terms and conditions governing this Note, including the terms
and conditions under which this Note may be prepaid or its maturity date
accelerated.

         This Note is secured pursuant to the Collateral Documents, as more
specifically described in the Agreement, and reference is made thereto for a
statement of the terms and provisions thereof.

         This Note is made, executed and delivered by the Borrower and delivered
to the Lender in increase and replacement of and substitution for that certain
Revolving Note dated as of November 30, 1999, executed by the Borrower payable
to the order of the Lender in the stated principal amount of $2,750,000 (the
"Prior Note"). All liens and security interests in Property securing payment of
the Prior Note shall continue in full force and effect, uninterrupted and
unabated, as security for payment of this Note.


                                         XETA TECHNOLOGIES, INC.,
                                         an Oklahoma corporation

                                         By: /s/ ROBERT B. WAGNER
                                            -----------------------------------
                                         Print Name: Robert B. Wagner
                                                    ---------------------------
                                         Title:  VP Finance
                                               --------------------------------





<PAGE>   7


                   SCHEDULE OF LOANS AND PAYMENTS OF PRINCIPAL
                                       TO
                    REVOLVING NOTE OF XETA TECHNOLOGIES, INC.
                              DATED AUGUST 21, 2000


<TABLE>
<CAPTION>
                           Principal             Maturity               Principal
                           Amount of           of Interest               Amount           Unpaid
       Date                  Loan                Period                   Paid            Balance
--------------------    ---------------     -----------------      -----------------     ---------
<S>                     <C>                 <C>                    <C>                   <C>
Balance Forward
</TABLE>