<PAGE>
                                                                   Exhibit 10.15

                       THIRD AMENDMENT OF CREDIT AGREEMENT

         THIS THIRD AMENDMENT OF CREDIT AGREEMENT (this "AMENDMENT") is entered
into on December 21, 2000, effective as of January 2, 2001, between PROTECTION
ONE ALARM MONITORING, INC., a Delaware corporation ("BORROWER"), each of the
Persons which is a signatory to this Amendment (collectively, "LENDERS"), and
WESTAR INDUSTRIES, INC., as Administrative Agent for the Lenders (in such
capacity, together with its successors in such capacity, "ADMINISTRATIVE
AGENT").

                                 R E C I T A L S

         A. Borrower, Lenders and Administrative Agent entered into the Credit
Agreement dated as of December 21, 1998 (as renewed, extended, modified, and
amended from time to time, the "CREDIT AGREEMENT"; capitalized terms used herein
shall, unless otherwise indicated, have the respective meanings set forth in the
Credit Agreement), providing for a revolving credit facility in the original
maximum principal amount of $500,000,000.

         B. Pursuant to a letter agreement dated as of September 30, 1999,
Borrower reduced the Total Commitment to $250,000,000.

         C. The Lenders and the Administrative Agent entered into that certain
Assignment and Acceptance dated December 17, 1999 wherein the Administrative
Agent and the Lenders assigned all of their rights and obligations under the
Credit Agreement to Westar Industries, Inc. (f/k/a Westar Capital, Inc.).

         D. Borrower, Lender and Administrative Agent entered into a Second
Amendment of Credit Agreement effective as of February 29, 2000 pursuant to
which certain provisions of the Credit Agreement were amended.

         E. Borrower, Lender, and Administrative Agent desire to further modify
certain provisions contained in the Credit Agreement, subject to the terms and
conditions set forth herein.

         NOW, THEREFORE, for good and valuable consideration, the receipt and
sufficiency of which are hereby acknowledged, Borrower, Lender, and
Administrative Agent agree as follows:

         1. AMENDMENTS TO THE CREDIT AGREEMENT. SECTION 1.1 is hereby amended to
delete the definitions of "APPLICABLE MARGIN" and "TERMINATION DATE" in their
entirety and replace such definitions with the following:

               APPLICABLE MARGIN means, as of any date of determination, the
         interest margin over Base Rate or the Eurodollar Rate, as the case may
         be, that corresponds to the Leverage Ratio set forth below on such date
         of determination:
<PAGE>

<TABLE>
<CAPTION>
=============  ================================  =========================  =================  ====================
                                                        APPLICABLE             APPLICABLE          APPLICABLE
                                                          MARGIN               MARGIN FOR          MARGIN FOR
   LEVEL                LEVERAGE RATIO                FOR BASE RATE            EURODOLLAR          COMMITMENT
                                                        BORROWINGS             BORROWINGS             FEES
=============  ================================  =========================  =================  ====================
<S>            <C>                               <C>                        <C>                <C>
     1         Less than or equal to 5.00:1               3.00%                  4.00%               0.375%
-------------  --------------------------------  -------------------------  -----------------  --------------------
     2         Greater than 5.00:1 but less               3.25%                  4.25%                0.50%
               than or equal to 5.25:1
-------------  --------------------------------  -------------------------  -----------------  --------------------
     3         Greater than 5.25:1 but less               3.75%                  4.75%                0.50%
               than or equal to 5.50:1
-------------  --------------------------------  -------------------------  -----------------  --------------------
     4         Greater than 5.50:1                        4.25%                  5.25%                0.50%
=============  ================================  =========================  =================  ====================
</TABLE>

               The Applicable Margin payable by the Borrower on the
         Borrowings outstanding hereunder shall be adjusted on the date of
         receipt by the Administrative Agent of the Financial Statements and
         Compliance Certificates required to be delivered pursuant to SECTIONS
         9.3(a) AND (b) as tested using the Leverage Ratio for the most recent
         fiscal quarter. If the Financial Statements and Compliance Certificates
         required pursuant to SECTION 9.3(a) OR (b) are not received by the
         Administrative Agent by the date required, the Applicable Margin shall
         be determined as if the Leverage Ratio is greater than 5.50:1. From the
         date hereof until the Borrower's Financial Statements for the fiscal
         quarter ended March 31, 2001, and corresponding Compliance Certificate
         are delivered pursuant to SECTION 9.3(b), the Applicable Margin shall
         be determined based on Level 1.

               TERMINATION DATE means the earlier of (a) March 2, 2001, and
         (b) the effective date of any other termination or cancellation of
         Lenders' commitments to lend under, and in accordance with, this
         Agreement.

         2. AMENDMENT OF CREDIT AGREEMENT AND OTHER LOAN DOCUMENTS. All
references in the Loan Documents to the Credit Agreement shall henceforth
include references to the Credit Agreement as modified and amended by this
Amendment, and as may, from time to time, be further modified, amended,
restated, extended, renewed, and/or increased.

         3. RATIFICATIONS. Borrower (a) ratifies and confirms all provisions of
the Loan Documents as amended by this Amendment, (b) ratifies and confirms that
all guaranties, assurances, and Liens, if any, granted, conveyed, or assigned to
the Credit Parties under the Loan Documents are not released, reduced, or
otherwise adversely affected by this Amendment and continue to guarantee,
assure, and secure full payment and performance of the present and future
Obligation, and (c) agrees to perform such acts and duly authorize, execute,
acknowledge, deliver, file, and record such additional documents, and
certificates as the Credit Parties may reasonably request in order to create,
perfect, preserve, and protect those guaranties, assurances, and Liens.


                                       2
<PAGE>

         4. REPRESENTATIONS. Borrower represents and warrants to the Credit
Parties that as of the date of this Amendment: (a) this Amendment has been duly
authorized, executed, and delivered by Borrower and each of the other Obligors
that are parties to this Amendment; (b) no action of, or filing with, any
Governmental Authority is required to authorize, or is otherwise required in
connection with, the execution, delivery, and performance by Borrower or any
other Obligor of this Amendment; (c) the Loan Documents, as amended by this
Amendment, are valid and binding upon Borrower and the other Obligors and are
enforceable against Borrower and the other Obligors in accordance with their
respective terms, except as limited by Debtor Relief Laws and general principles
of equity; (d) the execution, delivery, and performance by Borrower and the
other Obligors of this Amendment do not require the consent of any other Person
and do not and will not constitute a violation of any Governmental Requirement,
order of any Governmental Authority, or material agreements to which Borrower or
any other Obligor is a party thereto or by which Borrower or any other Obligor
is bound; (e) all representations and warranties in the Loan Documents are true
and correct in all material respects on and as of the date of this Amendment,
except to the extent that (i) any of them speak to a different specific date, or
(ii) the facts on which any of them were based have been changed by transactions
contemplated or permitted by the Credit Agreement; and (f) both before and after
giving effect to this Amendment, no Potential Default or Default exists.

         5. CONDITIONS. This Amendment shall not be effective unless and until:

         (a) this Amendment has been executed by Borrower, the other Obligors,
Administrative Agent, and the Required Lenders;

         (b) Borrower shall have delivered to Administrative Agent such
documents satisfactory to Administrative Agent evidencing the authorization and
execution of this Agreement, and the other documents executed and delivered in
connection herewith (collectively, the "AMENDMENT DOCUMENTS"); and

         (c) Borrower shall have paid to Administrative Agent, for the account
of the Credit Parties as Administrative Agent shall determine, (i) an amendment
fee in an amount equal to .25% of the Total Commitment on the effective date of
this Amendment ($287,500), which shall be credited against any fee hereafter
paid in connection with any long-term facility negotiated with the Lender, and
(ii) the reasonable fees and expenses of Administrative Agent's counsel
(including the allocated costs of internal counsel).

         6. CONTINUED EFFECT. Except to the extent amended hereby or by any
documents executed in connection herewith, all terms, provisions, and conditions
of the Credit Agreement and the other Loan Documents, and all documents executed
in connection therewith, shall continue in full force and effect and shall
remain enforceable and binding in accordance with their respective terms.

         7. MISCELLANEOUS. Unless stated otherwise (a) the singular number
includes the plural and vice versa and words of any gender include each other
gender, in each case, as appropriate, (b) headings and captions may not be
construed in interpreting


                                       3
<PAGE>

provisions, (c) this Amendment shall be construed -- and its performance
enforced -- under Texas law, (d) if any part of this Amendment is for any reason
found to be unenforceable, all other portions of it nevertheless remain
enforceable, and (e) this Amendment may be executed in any number of
counterparts with the same effect as if all signatories had signed the same
document, and all of those counterparts must be construed together to constitute
the same document.

         8. PARTIES. This Amendment binds and inures to Borrower and the Credit
Parties and their respective successors and permitted assigns.

         9. ENTIRETIES. THE CREDIT AGREEMENT AND THE OTHER LOAN DOCUMENTS, AS
AMENDED BY THIS AMENDMENT AND THE OTHER AMENDMENT DOCUMENTS, REPRESENT THE FINAL
AGREEMENT BETWEEN THE PARTIES ABOUT THE SUBJECT MATTER OF THE CREDIT AGREEMENT
AND MAY NOT BE CONTRADICTED BY EVIDENCE OF PRIOR, CONTEMPORANEOUS, OR SUBSEQUENT
ORAL AGREEMENTS OF THE PARTIES. THERE ARE NO UNWRITTEN ORAL AGREEMENTS BETWEEN
THE PARTIES.


            [THE REMAINDER OF THIS PAGE IS INTENTIONALLY LEFT BLANK]


                                       4
<PAGE>

                      SIGNATURE PAGE TO THIRD AMENDMENT OF
                             CREDIT AGREEMENT AMONG
               PROTECTION ONE ALARM MONITORING, INC., AS BORROWER,
                WESTAR INDUSTRIES, INC., AS ADMINISTRATIVE AGENT
                                       AND
                            THE LENDERS NAMED HEREIN

         EXECUTED on and effective as of the dates first above written.

                      PROTECTION ONE ALARM MONITORING, INC., a Delaware
                      corporation, as Borrower


                      By:       /s/ Anthony D. Somma
                         -------------------------------------------------------
                         Name:  Anthony D. Somma
                              --------------------------------------------------
                         Title: Chief Financial Officer, Secretary and Treasurer
                               -------------------------------------------------


<PAGE>

                      SIGNATURE PAGE TO THIRD AMENDMENT OF
                             CREDIT AGREEMENT AMONG
               PROTECTION ONE ALARM MONITORING, INC., AS BORROWER,
                WESTAR INDUSTRIES, INC., AS ADMINISTRATIVE AGENT
                                       AND
                            THE LENDERS NAMED HEREIN

         EXECUTED on and effective as of the dates first above written.

                          WESTAR INDUSTRIES, INC., as Administrative Agent
                          and a Lender


                          By:       /s/ Paul R. Geist
                             ---------------------------------------------------
                             Name:  PAUL R. GEIST
                                  ----------------------------------------------
                             Title: President
                                   ---------------------------------------------


<PAGE>


         To induce the Credit Parties to enter into this Amendment, each of the
undersigned (a) consents and agrees to the Amendment Documents' execution and
delivery, (b) ratifies and confirms that all guaranties, assurances, and Liens,
if any, granted, conveyed, or assigned to the Credit Parties under the Loan
Documents are not released, diminished, impaired, reduced, or otherwise
adversely affected by the Amendment Documents and continue to guarantee, assure,
and secure the full payment and performance of all present and future
Obligations (except to the extent specifically limited by the terms of such
guaranties, assurances, or Liens), (c) agrees to perform such acts and duly
authorize, execute, acknowledge, deliver, file, and record such additional
guaranties, assignments, security agreements, deeds of trust, mortgages, and
other agreements, documents, instruments, and certificates as the Credit Parties
may reasonably deem necessary or appropriate in order to create, perfect,
preserve, and protect those guaranties, assurances, and Liens, and (d) waives
notice of acceptance of this consent and agreement, which consent and agreement
binds the undersigned and its successors and permitted assigns and inures to the
Credit Parties and their respective successors and permitted assigns.

         EXECUTED on and effective as of the dates first above written.

                       PROTECTION ONE, INC., a Delaware corporation

                       By:      /S/ ANTHONY D. SOMMA
                          -----------------------------------------------------
                          Name: Anthony D. Somma
                          -----------------------------------------------------
                          Title:Chief Financial Officer, Secretary and Treasurer
                          -----------------------------------------------------

                       NETWORK MULTI-FAMILY SECURITY CORPORATION, a Delaware
                       corporation


                       By:       /S/ ANTHONY D. SOMMA
                          ------------------------------------------------------
                          Name:  Anthony D. Somma
                          ------------------------------------------------------
                          Title: Assistant Secretary and Assistant Treasurer
                          ------------------------------------------------------