<PAGE>   1
                                                                   EXHIBIT 10.12






                               FIRST AMENDMENT TO

                                 LOAN AGREEMENT

                      $40,000,000 REVOLVING LINE OF CREDIT

                                      from

                             BANK ONE, COLORADO, NA,

                                  COBANK, ACB,

                                       and

                                INTRUST BANK, NA

                                       to

                                  T-NETIX, INC.

                                  July 11, 2000


<PAGE>   2


                                TABLE OF CONTENTS

<TABLE>
<CAPTION>
                                                                                                         Page
                                                                                                         ----
<S>           <C>                                                                                        <C>
ARTICLE 1.    DEFINITIONS...................................................................................1
      1.1     Definitions...................................................................................1

ARTICLE 2.    AMENDMENTS....................................................................................3
      2.1     Amendment to Section 2.5(b).  ................................................................3
      2.2     Amendment to Section 2.5(c)...................................................................4
      2.3     Amendment to Section 6.11.....................................................................4
      2.4     Addition of Section 6.18......................................................................5
      2.5     Addition of Section 6.19.  ...................................................................5

ARTICLE 3.    MISCELLANEOUS.................................................................................6
      3.1     Extension Fee.................................................................................6
      3.2     Interest Rate Effective Date..................................................................6
      3.3     Counterpart Execution.........................................................................6
      3.4     Inconsistent Provisions; Severability.........................................................6
      3.5     References and Titles.........................................................................6
      3.6     Successors and Assigns........................................................................6
      3.7     Confirmation of Original Loan Agreement.  ....................................................6
</TABLE>



                                        i
<PAGE>   3


                        FIRST AMENDMENT TO LOAN AGREEMENT

         THIS FIRST AMENDMENT TO LOAN AGREEMENT (this "First Amendment"), dated
as of July 11, 2000, is by and among, T-NETIX, INC. a Colorado corporation
("Borrower"), Lenders (as defined in the Original Loan Agreement (as hereinafter
defined)) and BANK ONE, COLORADO, NA, a national banking association, as agent
for the Lenders (in such capacity, the "Agent").

         The parties executed and delivered the Loan Agreement dated
September 9, 1999 (the "Original Loan Agreement"). Section 12.5 of the Original
Loan Agreement permits such agreement to be amended with the consent of the
Lenders. The Lenders have waived certain Events of Default through December 31,
1999, under the Original Loan Agreement and the parties agreed to make certain
amendments to the Original Loan Agreement in consideration of such waiver.

         In consideration of the mutual covenants set forth in this First
Amendment and the other Loan Documents, and 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 in their entirety to
read as follows:

         "Base Rate" means a per-annum interest rate equal to the Prime Rate
plus the Base Rate Spread. The Base Rate is adjustable the day of any change in
the Prime Rate, regardless of whether Borrower has notice of such change.

         "LIBOR Spread" means a percentage, which shall be adjusted five
Business Days after each receipt by Agent of the Quarterly Compliance
Certificate for the applicable Quarter as required by Section 6.2 below,
determined by reference to the following table based on a ratio of Senior Bank
Debt to Unadjusted EBITDA.


<PAGE>   4


<TABLE>
<CAPTION>
                          Ratio of Senior Bank Debt
                            to Unadjusted EBITDA                        LIBOR Spread
                          -------------------------                     ------------
                          <S>                                           <C>
                              > 3.61 and < 3.85                             4.00%

                              > 3.26 and < 3.60                             3.25%

                              > 2.76 and < 3.25                             2.75%

                              > 2.26 and < 2.75                             2.25%

                              > 1.51 and < 2.25                             2.00%

                                         < 1.50                             1.75%
</TABLE>

         "Maturity Date" means the earlier of (i) acceleration of the Loan or
(ii) April 30, 2001, subject to extensions as provided in Section 2.1(h) of the
Original Loan Agreement.

         "Maximum Loan Amount" means $40,000,000, minus the Letters of Credit
Liability. Notwithstanding the foregoing, the Maximum Loan Amount from the date
of execution hereof until August 14, 2000, will be $30,526,650. Thereafter, the
Maximum Loan Amount shall be equal to no more than the amount equal to the
product obtained by multiplying Unadjusted EBITDA as of the end of the Quarter
ending June 30, 2000, and each Quarter thereafter by the ratio for such Quarter
of Senior Bank Debt to Unadjusted EBITDA for such Quarter. The calculation of
the Maximum Loan Amount shall be made by the Agent from time to time on the last
day that financial statements for the Quarter or year-end are required to be
delivered by the Borrower to the Agent in accordance with Section 6.2(a) and (b)
of the Agreement (the "Effective Date"). The Maximum Loan Amount so calculated
shall remain in effect until the next Effective Date.

         The following definitions shall be added to the Agreement.

         "Agreement" means, collectively, the Original Loan Agreement and the
First Amendment.

         "Base Rate Spread" means a percentage, which shall be adjusted five
Business Days after each receipt by the Agent of the Quarterly Compliance
Certificate for the applicable Quarter, as required by Section 6.2 of the
Original Loan Agreement, determined by reference to the following table:


                                       2
<PAGE>   5


<TABLE>
<CAPTION>
                          Ratio of Senior Bank Debt
                            to Unadjusted EBITDA                        Prime Spread
                          -------------------------                     ------------
                          <S>                                           <C>
                              > 3.61 and < 3.85                             1.25%

                              > 3.26 and < 3.60                             0.75%

                              > 2.76 and < 3.25                             0.50%

                              > 2.26 and < 2.75                             0.25%

                              > 1.51 and < 2.25                                0%

                                         < 1.50                                0%
</TABLE>

         "Extension Fee" means an amount equal to $50,000 payable on the date of
the execution and delivery of this First Amendment.

         "First Amendment" means this First Amendment to Loan Agreement dated as
of May 26, 2000, among the Borrower, the Lenders and the Agent.

         "Original Loan Agreement" means the Loan Agreement dated as of
September 9, 1999, among the Borrowers, the Lenders and the Agent.

         "Senior Bank Debt" means the Indebtedness of the Borrower incurred
pursuant to the Agreement.

         "Subordinated Indebtedness" means the Subordinated Loan Agreement
between the Borrower and Daniel M. Carney dated as of April 14, 2000, as amended
or supplemented from time to time.

         "Tangible Stockholders' Equity" means the total stockholder's equity,
less intangibles, less deferred tax asset, plus the balance of the redeemable
preferred stock and the principal amount outstanding of the Subordinated
Indebtedness, each as shown on the most recent financial statements of the
Borrower delivered to the Agent in accordance with Section 6.2 of the Original
Loan Agreement.

                                   ARTICLE 2.
                                   AMENDMENTS

         2.1 Amendment to Section 2.5(b). Section 2.5(b) of the Original Loan
Agreement shall be amended to read as follows: "Unused Fee." Borrower shall pay
to the Agent on behalf of the Lenders a Quarterly unused fee equal to the
product obtained by multiplying the Unused Availability by the Unused Fee
Percentage, which fee is payable in arrears within thirty days of the end of
each Quarter. The Unused Fee will be divided among the Lenders pro rata in
accordance with each Lender's Percentage Interest. "Unused Availability" shall
be calculated within thirty days after the end of each Quarter (and on a
pro-rata basis for any partial Quarter) and means the amount by which (i) the
Maximum Loan Amount exceeds (ii) the average daily outstanding balance of the
Loan during such Quarter as reasonably determined by the Agent.


                                       3
<PAGE>   6


The "Unused Fee Percentage" shall be determined within five days of the end of
the applicable Quarter by reference to the following table based on the ratio of
Senior Bank Debt to Unadjusted EBITDA:

<TABLE>
<CAPTION>
                   Ratio of Senior Bank Debt                      Unused Fee Percentage
                     to Unadjusted EBITDA                         (on an annual basis)
                   -------------------------                      ---------------------
                   <S>                                            <C>
                       > 3.61 and < 3.85                                 .400 %

                       > 3.26 and < 3.60                                 .350 %

                       > 2.76 and < 3.25                                 .300 %

                       > 2.26 and < 2.75                                 .225 %

                       > 1.51 and < 2.25                                 .150 %

                                  < 1.50                                 .100 %"
</TABLE>


         2.2 Amendment to Section 2.5(c). Section 2.5(c) of the Original Loan
Agreement shall be amended to read as follows: "Agent's Fee. $25,000 annually,
due and payable on or before September 30 of each year, commencing September 30,
2000."

         2.3 Amendment to Section 6.11. Section 6.11 of the Original Loan
Agreement shall be amended to read as follows: "Borrower shall comply with the
following financial covenants:

             (1) Senior Bank Debt to Unadjusted EBITDA. Borrower shall maintain
a ratio of Senior Bank Debt to Unadjusted EBITDA less than or equal to the ratio
set forth below for the period in which the applicable Quarter ends, calculated
at the end of each Quarter and unless otherwise specified based on such Quarter
and the three immediately preceding Quarters:

<TABLE>
<CAPTION>
                         Period                                      Ratio
                         ------                                      -----
                         <S>                                         <C>
                         6/30/2000                                   3.75

                         9/30/2000                                   3.65

                         12/31/2000                                  3.45

                         3/31/2001, and at the end of each           3.25
                                           Quarter thereafter.
</TABLE>


             (2) Fixed Charge Coverage Ratio. Borrower shall maintain a Fixed
Charge Coverage Ratio not less than the ratio set forth below for the period in
which the applicable Quarter ends, calculated at the end of each Quarter and
unless otherwise specified based on such Quarter and the three immediately
preceding Quarters:



                                       4
<PAGE>   7


<TABLE>
<CAPTION>
                         Period                                      Ratio
                         ------                                      -----
                         <S>                                         <C>
                         6/30/2000                                   2.85

                         9/30/2000                                   3.00

                         12/31/2000                                  3.25

                         3/31/2001, and at the end of each           3.50
                                           Quarter thereafter.
</TABLE>


             (3) Minimum EBITDA. Borrower shall maintain a minimum EBITDA at the
end of the Quarter identified below in the amount set forth below for such
Quarter (as shown in the applicable financial statements):

<TABLE>
                        <S>                                               <C>
                        6/30/2000                                         $1,600,000

                        9/30/2000                                         $2,300,000

                        12/31/2000                                        $3,500,000

                        3/31/2001, and each Quarter                       $4,000,000
                                        thereafter."
</TABLE>

         2.4 Addition of Section 6.18. Article VI of the Original Loan Agreement
shall be amended by the addition of the following: "Section 6.18. Tangible
Shareholders' Equity. Borrower shall maintain a minimum Tangible Shareholders'
Equity at the end of each Quarter identified below in the amount set forth below
for such Quarter:

<TABLE>
                        <S>                                              <C>
                        6/30/2000                                        $10,115,000

                        9/30/2000                                         $9,500,000

                        12/31/2000                                        $9,975,000

                        3/31/2001, and each Quarter                       $9,975,000
                                        thereafter."
</TABLE>


         2.5 Addition of Section 6.19. Article VI of the Original Loan Agreement
shall be amended by the addition of the following: "6.19 Provisions Regarding
Subordinated Indebtedness. The Borrower agrees so long as the Obligations remain
outstanding that it will not consent to any amendment of the Subordinated
Indebtedness without the prior written consent of the Agent. The Borrower
further agrees that it will make no payment or prepayment of the


                                       5
<PAGE>   8


principal of the Subordinated Indebtedness prior to April 30, 2001. If the
Maturity Date is extended in accordance with the Original Loan Agreement, the
Agent, on behalf of the Lenders, may, in its sole discretion, agree to an
amortization schedule for the Subordinated Indebtedness."

                                   ARTICLE 3.
                                  MISCELLANEOUS

         3.1 Extension Fee. On or prior to the date of the delivery hereof,
Borrower shall pay to Agent on behalf of Lenders the Extension Fee and all other
costs, including legal costs, in connection with the execution and delivery of
this First Amendment.

         3.2 Interest Rate Effective Date. The parties agree that the
definitions of Base Rate, Libor Spread and Base Rate Spread are effective as of
June 1, 2000.

         3.3 Counterpart Execution. This First Amendment may be executed in any
number of counterparts which together will be one and the same instrument. This
First Amendment shall become effective whenever each party shall have signed at
least one counterpart.

         3.4 Inconsistent Provisions; Severability. In case of any
irreconcilable conflict between the provisions of this First Amendment and those
of the Loan Documents and the Notes, the provisions of this First 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.

         3.5 References and Titles. All references in this First Amendment to
Exhibits, Schedules, sections and subsections and other subdivisions refer to
the Exhibits, Schedules, sections and subsections and other subdivisions of this
First 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 "this Agreement," "this instrument," "herein," "hereof," "hereby,"
"hereunder" and words of similar import refer to this First 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.

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

         3.7 Confirmation of Original Loan Agreement. Except as amended hereby,
the Original Loan Agreement is ratified and confirmed.



                                       6
<PAGE>   9

                      REMAINDER OF PAGE INTENTIONALLY BLANK


         EXECUTED to be effective as of the day and year first above written.


                                     BORROWER:

                                     T-NETIX, INC., a Colorado corporation

                                     By:
                                        ----------------------------------
                                        John Giannaula
                                        Vice President


                                     SUBSIDIARIES:

                                     T-NETIX TELECOMMUNICATIONS
                                     SERVICES, INC., a Texas Corporation, f/n/a
                                     GATEWAY TECHNOLOGIES, INC.

                                     By:
                                        ----------------------------------
                                        John Giannaula
                                        Vice President


                                     T-NETIX MONITORING CORPORATION, a
                                     Colorado corporation

                                     By:
                                        ----------------------------------
                                        John Giannaula
                                        Treasurer


                                     SPEAKEZ, INC., a Colorado corporation

                                     By:
                                        ----------------------------------
                                        John Giannaula
                                        Vice President


                                     T-NETIX JMS CORPORATION, a Colorado
                                     corporation

                                     By:
                                        ----------------------------------
                                        John Giannaula
                                        Vice President

                                        7
<PAGE>   10



                                     LENDERS:

                                     BANK ONE, COLORADO, NA, a national
                                     banking association

                                     By:
                                        ----------------------------------
                                        Cecilia L. Smith
                                        Vice President


                                     COBANK, ACB

                                     By:
                                        ----------------------------------
                                        Teresa L. Fountain
                                        Assistant Corporate Secretary


                                     INTRUST BANK, NA, a national banking
                                     association

                                     By:
                                        ----------------------------------
                                        Robert T. Harmon
                                        Vice President


                                     AGENT:

                                     BANK ONE, COLORADO, NA, a national
                                     banking association

                                     By:
                                        ----------------------------------
                                        Cecilia L. Smith
                                        Vice President



                                        8