<PAGE>
                                                                   Exhibit 10.15




                              FOURTH AMENDMENT TO

                                 LOAN AGREEMENT

                     REVOLVING LINE OF CREDIT AND TERM LOAN

                                      FROM

                                 BANK ONE, NA,

                                  COBANK, ACB,

                                      AND

                                INTRUST BANK, NA

                                       TO

                                 T-NETIX, INC.

                                 APRIL 12, 2002
<PAGE>
                       FOURTH AMENDMENT TO LOAN AGREEMENT


      This Fourth Amendment to Loan Agreement ("FOURTH AMENDMENT"), dated as of
April 12, 2002, is by and among T-NETIX, INC., a Colorado corporation
("BORROWER"), Lenders (as defined in the Original Loan Agreement), and BANK ONE,
NA, a national banking association, f/k/a Bank One, Colorado, N.A., as agent for
Lenders (in such capacity, the "AGENT").

      The parties executed and delivered the original Loan Agreement, dated
September 9, 1999 ("ORIGINAL LOAN AGREEMENT"), which Original Loan Agreement was
amended pursuant to the First Amendment to Loan Agreement, dated July 11, 2000,
the Second Amendment to Loan Agreement dated April 16, 2001, and the Third
Amendment to Loan Agreement dated March 26, 2002. Section 12.5 of the Original
Loan Agreement permits such agreement to be amended with the consent of the
Lenders.

      In consideration of the mutual covenants set forth in this Fourth
Amendment and the other Loan Documents, and for other good and valuable
consideration, the receipt and sufficiency of which are hereby acknowledged,
Borrower, Lenders, and Agent agree as follows:

                                   ARTICLE 1.
                                   DEFINITIONS

      1.1   DEFINITIONS. Unless amended hereby, each capitalized term as used
herein shall have the meaning given it in Section 1.1 of the Original Loan
Agreement. The following definitions shall be amended and restated in their
entirety to read as follows:

            "Agreement" shall mean, collectively, the Original Loan Agreement,
            as amended by the First Amendment to Loan Agreement, the Second
            Amendment to Loan Agreement, the Third Amendment to Loan Agreement
            and this Fourth Amendment to Loan Agreement.

            "Loan" shall collectively mean a $14,000,000 revolving line of
            credit (the "REVOLVING LINE") and a $7,600,000 term loan (the "TERM
            LOAN"). Provided however, in the event Borrower exercises its Option
            to Extend Maturity Date pursuant to Section 2.6, the Revolving Line
            shall be reduced to $11,500,000 and the Term Loan shall be reduced
            to $7,200,000.

            "Maturity Date" shall mean the earlier of (i) the date of
            acceleration of the Loan, or (ii) June 30, 2002 (unless extended
            pursuant to Borrower's exercise of its Option to Extend Maturity
            Date pursuant to Section 2.6).

            "Maximum Loan Amount" shall mean $21,600,000, comprised of the
            Revolving Line and the Term Loan, which amount shall be reduced to
            $18,700,000 in the event Borrower exercises its Option to Extend
            Maturity


                                       2
<PAGE>
            Date pursuant to Section 2.6. Notwithstanding Section 2.1 of the
            Original Loan Agreement, Borrower may not borrow, repay and reborrow
            on that portion of the Loan constituting the Term Loan.

      The following new definitions are hereby added to Section 1.1:

            "Option to Extend Maturity Date" shall mean Borrower's conditional
            option to extend the Maturity date as set forth in Section 2.6.

            "Option Exercise Date" shall mean June 26, 2002; the latest date by
            which Borrower must provide its notice to exercise Option to Extend
            Maturity Date.


                                    ARTICLE 2
                              CONDITIONS PRECEDENT

      2.1   Conditions Precedent The following conditions precedent must be
strictly fulfilled, substantial performance being insufficient, on or before
April 12, 2002 (the "Amendment Closing Date") and the failure of any one of
which to be fulfilled shall be deemed an Event of Default:

            (a)   Borrower shall pay to Lenders an Amendment Fee equal to
      one-half of one percent (.50%) of the Maximum Loan Amount on the date
      hereof to be immediately distributed to Lenders on a pro rata basis by
      Agent.

            (b)   Borrower shall pay all accrued interest to date through and
      including the Amendment Closing Date, due on the Loan at the Interest Rate
      payable to each Lender on Lender's Percentage Interest (as defined in the
      Loan Agreement) in effect immediately prior to the date of this Fourth
      Amendment;

            (c)   In the event the outstanding balance of the Loan on the
      Amendment Closing Date exceeds the Maximum Loan Amount (as defined in this
      Fourth Amendment), Borrower shall make such payment as is necessary to
      reduce said balance to an amount that is less than or equal to the Maximum
      Loan Amount;

            (d)   Borrower shall have paid all fees and expenses of Agent and
      Lenders incurred in relation to this Fourth Amendment;

            (e)   Borrower shall have performed all of its Obligations under the
      Loan, including without limitation, (i) shall be current in all payments
      of principal and interest, (ii) shall have submitted to Agent all
      financial reports and Covenant Compliance Certificates, (iii) shall be in
      compliance with all Covenants governed by the effective Loan Agreement.,
      and (iv) shall have submitted to agent "pre-audit" results for fiscal year
      2001;


                                       3
<PAGE>
            (f)   Agent shall have received evidence acceptable to Agent of
      appropriate corporate action on the part of Borrower and Subsidiaries
      hereon authorizing the execution and delivery of this Fourth Amendment;

            (g)   Agent shall have received opinions of Borrowers' counsel in
      form and substance acceptable to Agent


                                    ARTICLE 3
                                   AMENDMENTS

      3.1   Subordinated Indebtedness. The Subordinated Loan Agreement between
Borrower and Daniel M. Carney, dated as of April 14, 2000, as amended or
supplemented from time to time shall be renewed and extended by a written
extension of the subordinated debt under the same terms as the previous year to
mature no sooner than February 3, 2003.

      3.2   Repayment Terms. (a) Payments of interest on the Revolving Line are
due on the first day of each month during the remaining term of the Loan and
shall continue to be due on the first day of each month following an exercise by
Borrower of its Option to Extend Maturity Date. The entire outstanding principal
balance together with accrued and unpaid interest on the Revolving Loan shall be
due and payable on the Maturity Date. Amounts available to be drawn under the
Revolving Line shall be reduced by $300,000 per month commencing August 1, 2002,
and on the first day of each month thereafter. If the outstanding principal
balance of the Revolving Line on the date of each such reduction exceeds the
amount available to be drawn under the Revolving Line, Borrower shall, within 3
days after notice from Agent, make principal payments in an amount necessary to
reduce the outstanding principal balance to the amount then available under the
Revolving Line. (b) Payments of $200,000 principal plus interest on the Term
Loan are due on the first day of each month and shall continue to be due on the
first day of each month following an exercise by Borrower of its Option to
Extend Maturity Date. The entire outstanding principal balance together with
accrued and unpaid interest on the Term Loan shall be due and payable on the
Maturity Date.

      3.3   A new Section 2.6 is hereby added to the Loan Agreement as follows:

                  2.6   Option to Extend Maturity Date. Upon satisfaction of the
            conditions set forth in this Section 2.6 provided no Event of
            Default exists nor any event which with the passage of time or
            giving of notice, or both, would constitute an Event of Default,
            Borrower may elect to extend the Maturity Date to January 3, 2003.

                  (a)   Borrower shall provide written notice to Agent of its
            election to exercise its Option to Extend Maturity Date ("Exercise
            Notice"). Said Exercise Notice shall be in the form attached hereto
            as Exhibit A. The Exercise Notice must be received by Agent not
            earlier than June 19, 2002 and not later than 3:00 p.m. Central
            Daylight Time June 26, 2002.


                                       4
<PAGE>
                  (b)   The Exercise Notice shall be accompanied by a payment to
            Agent of a fee in the amount of three percent (3%) of the Maximum
            Loan Amount on the date of the Exercise Notice (the "Option Fee").
            The Option Fee shall be held by Agent in an interest bearing deposit
            account for the benefit of the Lenders. The amount of initial
            deposit of the Option Fee is referred to hereafter as the "
            Beginning Option Fee Balance". The Option Fee shall be refunded to
            Borrower or distributed pro rata to Lenders, as the case may be, in
            accordance with the following:

                        (i)   If the entire outstanding principal balance of the
                  Loan plus accrued and unpaid interest and fees has been repaid
                  to Lenders on or before July 31, 2002, Borrower shall receive
                  a refund of the entire Beginning Option Fee Balance, plus any
                  accrued interest. If Borrower shall fail to pay the entire
                  outstanding principal balance plus accrued and unpaid interest
                  and fees on or before July 31, 2002, Agent shall distribute to
                  each Lender on August 1, 2002, its pro rata shares of 50% of
                  the Beginning Option Fee Balance; or,

                        (ii)  If the entire outstanding principal balance of the
                  Loan plus accrued and unpaid interest and fees has been repaid
                  to Lenders on or before August 29, 2002, Borrower shall
                  receive a refund equal to 50% of the Beginning Option Fee
                  Balance, plus any accrued interest. If Borrower shall fail to
                  pay the entire outstanding principal balance plus accrued and
                  unpaid interest and fees on or before August 29, 2002, Agent
                  shall distribute to each Lender on August 30, 2002, its pro
                  rata shares of 33.33% of the Beginning Option Fee Balance; or,

                        (iii) If the entire outstanding principal balance of the
                  Loan plus accrued and unpaid interest and fees has been repaid
                  to Lenders on or before September 30, 2002, Borrower shall
                  receive a refund equal to 16.67% of the Beginning Option Fee
                  Balance, plus any accrued interest. If Borrower shall fail to
                  pay the entire outstanding principal balance plus accrued and
                  unpaid interest and fees on or before September 30, 2002,
                  Agent shall distribute to each Lender on October 1, 2002, its
                  pro rata shares of 16.67% of the Beginning Option Fee Balance.

      3.4   Financial Covenants. Section 6.11(c) of the Original Loan Agreement,
as amended, is further amended to provide as follows:

                  6.11  (c) Minimum EBITDA. Borrower shall maintain a minimum
            EBITDA commencing the Quarter ending June 30, 2002, and at the end
            of each Quarter thereafter of $4,500,000.

      3.5   Tangible Shareholder's Equity. Section 6.18 of the Original Loan
Agreement, as amended, is hereby amended to provided that Borrower shall
maintain a minimum Tangible


                                       5
<PAGE>
Shareholder's Equity for the Quarter ending June 30, 2002, and thereafter, of
$19,000,000.

      3.6   Notices. Section 12.2 of the Original Loan Agreement is hereby
amended by restating that section in its entirety as follows:

                  12.2  Notices. All notices given hereunder shall be in
            writing, shall be given by certified mail, return receipt requested,
            nationally recognized overnight courier service, telecopy, facsimile
            or copy delivered by hand, and, (i) if mailed, shall be deemed
            received three Business Days after having been deposited in a
            receptacle for United States mail, postage prepaid, (ii) if
            delivered by nationally recognized overnight air courier service,
            shall be deemed received one Business Day after having been
            deposited with such overnight air courier service, postage prepaid,
            and (iii) if delivered by telex, telecopy or hand delivery, shall be
            deemed received on the day the notice is sent (provided that the
            sender receives confirmation thereof), in each case addressed as
            follows (other than a notice of advance, which shall be sent to the
            Agent only):

            If to Borrower, or any Subsidiary to:

                                 T-NETIX, INC.
                                 2155 Chenault Drive, Suite 410
                                 Carrollton, Texas 75006
                                 Attention: Wayne A. Johnson II, General Counsel
                                 Facsimile: (303) 705-5584

                                 With a copy to:

                                 ______________________________________
                                 ______________________________________
                                 Attention:____________________________
                                 Facsimile:____________________________

            If to the Lenders, to:

                                 Bank One, NA
                                 Fourth Floor, Bank One Center
                                 1717 Main Street
                                 Dallas, TX 75201
                                 Attention: Hal E. Fudge
                                 Facsimile: (214) 290-2740


                                       6
<PAGE>
                                 With a copy to:

                                 Holme Roberts & Owen LLP
                                 1700 Lincoln, Suite 4100
                                 Denver, Colorado 80203
                                 Attention: Thomas L. DeVine, Esq.
                                 Facsimile: (303) 866-0200

            If to the Agent, to:

                                 Bank One, NA
                                 Fourth Floor, Bank One Center
                                 1717 Main Street
                                 Dallas, TX 75201
                                 Attention: Hal E. Fudge
                                 Facsimile: (214) 290-2740

                                 With a copy to:

                                 Holme Roberts & Owen LLP
                                 1700 Lincoln, Suite 4100
                                 Denver, Colorado 80203
                                 Attention: Thomas L. DeVine, Esq.
                                 Facsimile: (303) 866-0200

            Any party may, by written notice so delivered to the others, change
            the address or facsimile number to which delivery shall thereafter
            be made.


                                   ARTICLE 4.
                                 MISCELLANEOUS

      4.1   Counterpart/Facsimile Signatures. This Fourth Amendment may be
executed in any number of counterparts which together will be one and the same
instrument. This Fourth Amendment shall become effective whenever each party
shall have signed at least one counterpart. Facsimile signatures shall be
treated as original signatures hereon.

      4.2   Inconsistent Provisions; Severability. In case of any irreconcilable
conflict between the provisions of this Fourth Amendment and those of the Loan
Documents and the Notes, the provisions of this Fourth Amendment shall govern.
The invalidity, illegality or unenforceability of any provision of any of the
Loan Documents shall not in any way affect or impair the legality or
enforceability of the remaining provisions of each of the Loan Documents.

      4.3   References and Titles. All references in this Fourth Amendment to
Sections


                                       7
<PAGE>
and subsections and other subdivisions refers to the Sections and subsections
and other subdivisions of this Fourth Amendment unless expressly provided
otherwise. Headings are for convenience only and do not constitute any part of
such subdivisions and shall be disregarded in construing the language contained
in such subdivisions. The words "herein," "hereby," "hereunder" and words of
similar import refer to this Fourth Amendment as a whole and not to any
particular subdivision unless expressly so limited. Pronouns in masculine,
feminine and neuter genders shall be construed to include any other gender, and
words in the singular form shall be construed to include the plural and vice
versa, unless the context otherwise requires.

      4.4   Successors and Assigns. This Fourth Amendment shall be binding upon
and inure to the benefit of the parties hereto and their respective successors
and assigns.

      4.5   Confirmation of Original Loan Agreement. Except as amended hereby,
by the First Amendment to Loan Agreement, the Second Amendment to Loan
Agreement, the Third Amendment to Loan Agreement and this Fourth Amendment to
Loan Agreement, the Original Loan Agreement is ratified and confirmed.

      4.6   Release and Indemnity. Borrower agrees to release, indemnify, defend
and hold harmless the Agent and Lenders, all present and future officers,
directors, employees, attorneys and agents of the foregoing (the "Indemnitees")
from and against any claims, loss or damage to which any Indemnitee may be
subjected if any representation or warranty of Borrower proves to be incorrect
in any respect or as a result of any violation of any covenant and from and
against any and all other liabilities, losses, damages, penalties, judgments,
suits, claims, costs and expenses of any kind or nature whatsoever (including,
without limitation, the reasonable fees and disbursements of counsel) in
connection with the foregoing and any other investigative, administrative or
judicial proceedings, whether or not such Indemnitee shall be designated a party
thereto, which may be imposed on, incurred by or asserted against any such
Indemnitee, in any manner related to or arising out of or in connection with the
making of the Loan or the use or intended use of the proceeds of the Loan.


                    [Remainder of Page Intentionally Blank]


                                       8
<PAGE>
      EXECUTED to be effective as of the day and year first above written.

                                           "BORROWER"

                                           T-NETIX, INC., a Colorado corporation


                                           By:__________________________________
                                           Name:________________________________
                                           Title:_______________________________


                                           "SUBSIDIARIES"

                                           T-NETIX TELECOMMUNICATIONS SERVICES,
                                           INC., a Texas corporation, f/k/a
                                           GATEWAY TECHNOLOGIES, INC.


                                           By:__________________________________
                                           Name:________________________________
                                           Title:_______________________________


                                           T-NETIX MONITORING CORPORATION,
                                           a Colorado corporation


                                           By:__________________________________
                                           Name:________________________________
                                           Title:_______________________________


                                           SPEAKEZ, INC., a Colorado corporation


                                           By:__________________________________
                                           Name:________________________________
                                           Title:_______________________________


                                       9
<PAGE>
                                           T-NETIX CORPORATION, a Delaware
                                           corporation

                                           By:__________________________________
                                           Name:________________________________
                                           Title:_______________________________


                                           "LENDERS"

                                           BANK ONE, NA,
                                           a national banking association


                                           By:__________________________________
                                              Hal E. Fudge, First Vice-President


                                           COBANK, ACB


                                           By:__________________________________
                                           Name:________________________________
                                           Title:_______________________________


                                           INTRUST BANK, NA,
                                           a national banking association


                                           By:__________________________________
                                           Name:________________________________
                                           Title:_______________________________

                                           "AGENT"

                                           BANK ONE, NA,
                                           a national banking association

                                           By:__________________________________
                                              Hal E. Fudge, First Vice-President


                                       10
<PAGE>
                                    EXHIBIT A

                                 EXERCISE NOTICE


Bank One, NA
Fourth Floor, Bank One Center
1717 Main Street
Dallas, TX  75201
Attention:  Hal E. Fudge

Re:   Notice of Election to Exercise Option to Extend Maturity Date


Dear Mr. Fudge:

Pursuant to Section 2.6 of the Loan Agreement by and among the undersigned as
Borrower, and Bank One, NA, CoBank, ACB, and Intrust Bank, NA as Lenders, and
Bank One, NA as Agent, as such Loan Agreement has been amended, Borrower hereby
provides notice of its intent (this "Exercise Notice")to exercise its Option to
Extend Maturity Date and thereby extending the Maturity Date of the Loan to
January 3, 2003.

Borrower hereby represents and warrants to the Agent and each Lender that as of
the date of this Exercise Notice that: (i) it has performed all of its
Obligations pursuant to the Loan Agreement, (ii) it is in full compliance with
all covenants contained in the Loan Agreement, (iii) it has complied with all of
the conditions to the exercise of this Option to Extend Maturity Date as set
forth in the Fourth Amendment to Loan Agreement dated April 12, 2002, and (iv)
no Event of Default, nor any event which with the passage of time or giving of
notice would constitute an Event of Default exists or has occurred.

Borrower also represents and warrants to Lenders and Agent that on or before
June 26, 2002, it shall make payment to Agent in an amount equal to 3% of the
Maximum Loan Amount in effect as of the Option Exercise Date in accordance with
and subject to Section 2.6 (b) of the Amended Loan Agreement.

Sincerely,

T-Netix, Inc.



By:_________________________________
Its:________________________________


                                       11