<PAGE>

                                                                    EXHIBIT 4.12

THIS COMMON STOCK PURCHASE WARRANT HAS NOT BEEN REGISTERED UNDER THE SECURITIES
ACT OF 1933, AS AMENDED (THE "SECURITIES ACT") OR UNDER ANY APPLICABLE LAW OR
REGULATION OF ANY STATE.  THIS COMMON STOCK WARRANT MAY NOT BE SOLD, OFFERED,
ASSIGNED OR TRANSFERRED UNLESS THE WARRANT IS REGISTERED UNDER THE SECURITIES
ACT AND APPLICABLE STATE SECURITIES LAWS, OR UNLESS SUCH OFFERS, SALES,
ASSIGNMENTS AND TRANSFERS ARE MADE PURSUANT TO THE AVAILABLE EXEMPTIONS FROM THE
REGISTRATION REQUIREMENTS OF THOSE LAWS.

                      DIAL-THRU INTERNATIONAL CORPORATION

                         COMMON STOCK PURCHASE WARRANT

                                                       DATED:  December 22, 1999

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Number of Common Shares:  300,000              Holder:    Lawrence Vierra
Purchase Price:           $1.44
Expiration Date:          Three (3) years from date of vesting

     For identification only. The governing terms of this Warrant are set
                                 forth below.
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          DIAL-THRU INTERNATIONAL CORPORATION, a Delaware corporation (the
"Company"), hereby certifies that, for value received, Lawrence Vierra (the
"Holder") is entitled, subject to the terms set forth below, to purchase from
the Company at any time or from time to time prior to the earlier of (a) three
(3) years following the date of vesting of the Warrant or (b) the date of
termination of employment and resignation as a director of the Holder with the
Company (the "Exercise Period"), at the Purchase Price hereinafter set forth,
Three Hundred Thousand (300,000) fully paid and nonassessable shares of Common
Stock (as defined below) of the Company.  The number and character of such
shares of Common Stock and the Purchase Price are subject to adjustment as
provided herein.

          The purchase price per share of Common Stock issuable upon exercise of
this Warrant (the "Purchase Price") shall initially be $1.44, provided, however,
that the Purchase Price shall be adjusted from time to time as provided herein.

          As used herein the following terms, unless the context otherwise
requires, have the following respective meanings:

               (a)   The term "Company" means Dial-Thru International
          Corporation and any entity that shall succeed or assume the
          obligations of such corporation hereunder.

               (b)   The term "Common Stock" means the Company's common stock,
          .001 par value per share.

               (c)   The term "Fair Market Value" means the closing price of the
          shares of Common Stock on the date of delivery of any Notice of
          Exercise as reported on the Nasdaq SmallCap Market (or other exchange
          on which the Common Stock is traded) or,

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          if not then traded on any exchange, then the closing price as of such
          date on the over-the-counter market or, if not quoted on the over-the-
          counter market, then as determined by the Board of Directors.

               (d)   The term "Tax Withholding Liability" means all federal and
          state income taxes, social security taxes and other taxes applicable
          to compensation income arising from the exercise of this Warrant
          required by applicable law to be withheld by the Company.

               (e)   The term "Warrant Shares" means as of any date during the
          Exercise Period, that number of shares of Common Stock which shall be
          exercisable (subject to any vesting requirements) by the Holder hereof
          pursuant to the terms of this Warrant.

          1.   Vesting and Exercise of Warrant; Resale Restrictions.

               1.1.  Vesting. Holder's right to purchase 1/3 (100,000) of the
          Warrant Shares shall vest immediately upon the execution and delivery
          of this Warrant and shall be immediately exercisable. Holder's right
          to purchase 100,000 Warrant Shares hereunder shall vest upon the first
          and second anniversary dates of the date of this Agreement. Holder
          shall not have the right to acquire any Warrant Shares pursuant to
          this Warrant prior the vesting of such rights as set forth in this
          Section 1.1. Notwithstanding the vesting period set forth above,
          nothing contained herein shall be construed to imply that Holder is
          guaranteed employment with the Company for any specified period.

               1.2.  Method of Exercise. This Warrant may be exercised (subject
          to the vesting requirements set forth above) by the Holder hereof in
          whole or in part (but not as to a fractional share of Common Stock),
          at any time and from time to time during the Exercise Period for up
          to, but not more than, the number of vested Warrant Shares at such
          time, by delivery to the Company at its principal office of (i) a
          notice of exercise (a "Notice of Exercise") substantially in the form
          attached hereto as Exhibit A, (ii) evidence satisfactory to the
          Company of the authority of the person executing such Notice of
          Exercise, (iii) this Warrant, and (iv) payment of (A) the Purchase
          Price multiplied by the number of shares of Common Stock for which
          this Warrant is being exercised (the "Exercise Price") and (B) Tax
          Withholding Liability. Payment of the Exercise Price and Tax
          Withholding Liability shall be made by check or bank draft payable to
          the order of the Company or by wire transfer to the account of the
          Company. The shares so purchased shall be deemed to be issued as of
          the close of business on the date on which the Company shall have
          received from the Holder payment in full of the Exercise Price and Tax
          Withholding Liability and the other documents referred to herein (the
          "Exercise Date").

               1.3.  Regulation D Restrictions. The Holder hereof represents and
          warrants to the Company that it has acquired this Warrant and
          anticipates acquiring the shares of Common Stock issuable upon
          exercise of the Warrant solely for its own account for investment
          purposes and not with a view to or for distributing such securities
          unless such distribution has been registered with the Securities and
          Exchange Commission or an applicable exemption is available therefor.
          At the time this Warrant is exercised, the

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COMMON STOCK PURCHASE WARRANT - Page 2     (Dial-Thru International Corporation)
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          Company may require the Holder to state in the Notice of Exercise such
          representations concerning the Holder as are necessary or appropriate
          to assure compliance by the Holder with the Securities Act.

          2.   Delivery of Stock Certificates, etc., on Exercise. As soon as
practicable after the exercise of this Warrant, the Company will cause to be
issued in the name of and delivered to the Holder a certificate for the number
of fully paid and nonassessable shares of Common Stock to which the Holder shall
be entitled on such exercise, plus, in lieu of any fractional share to which the
Holder would otherwise be entitled, cash equal to such fraction multiplied by
the then applicable Purchase Price, together with any other stock or other
securities and property (including cash, where applicable) to which the Holder
is entitled upon such exercise pursuant to Section 1 or otherwise.

          3.   Adjustments on Certain Capital Transactions. On the occurrence of
any of the following events, the following adjustments to the rights granted
under this Warrant shall be made:

               3.1.  In case the number of outstanding shares of Common Stock of
          the Company shall be increased by way of a stock dividend, stock
          split, recapitalization, or other similar means, the number of
          unexercised shares of Common Stock covered by this Warrant shall be
          increased by the amount that a like number of shares of outstanding
          Common Stock shall have been increased as a result of such stock
          increase and the Purchase Price shall be adjusted by multiplying the
          Purchase Price in effect immediately prior to such stock increase by a
          fraction, the numerator of which shall be the number of unexercised
          shares covered by this Warrant immediately prior to such stock
          increase and the denominator of which shall be the number of
          unexercised shares of Common Stock covered by this Warrant as adjusted
          for such stock increase.

               3.2.  In case the number of outstanding shares of Common Stock of
          the Company shall be reduced by recapitalization, reverse stock split
          or otherwise, the number of unexercised shares covered by this Warrant
          shall be reduced by the amount that a like number of shares of
          outstanding Common Stock shall have been reduced as a result of such
          stock reduction and the Purchase Price shall be adjusted by
          multiplying the Purchase Price in effect immediately prior to such
          stock reduction by a fraction, the numerator of which shall be the
          number of unexercised shares covered by this Warrant immediately prior
          to such stock reduction and the denominator of which shall be the
          number of unexercised shares covered by this Warrant as adjusted for
          such stock reduction.

               3.3.  In case the Company shall consolidate with or merge into
          another corporation, the holder of this Warrant will thereafter
          receive, upon the exercise thereof in accordance with the terms of
          this Warrant, the securities or property to which the holder of the
          number of shares of Common Stock then deliverable upon the exercise of
          this Warrant would have been entitled upon such consolidation or
          merger ("Other Securities") and the Company shall take such steps in
          connection with such consolidation or merger as may be necessary to
          assure that the provisions hereof shall thereafter be applicable, as

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COMMON STOCK PURCHASE WARRANT - Page 3     (Dial-Thru International Corporation)
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          nearly as reasonably may be, in relation to any securities or property
          thereafter deliverable upon the exercise of this Warrant.

          4.   Rights as a Shareholder. Holder shall not have any rights as a
shareholder of the Company with respect to the shares subject to this Warrant.

          5.   Securities Law Requirements. Neither this Warrant nor the Warrant
Shares have been registered under the Securities Act or any state securities or
blue sky laws. Accordingly, upon (a) any transfer of this Warrant, any
transferee of this Warrant or (b) the exercise of this Warrant in whole or in
part, and if the Warrant Shares have not been registered under the Securities
Act, Holder or any other person exercising this Warrant shall, as applicable,
represent and agree in writing satisfactory to the Company that Holder or such
other person (a) is acquiring the shares for the purpose of investment and not
with a view to distribution thereof, (b) knows the shares have not been
registered under the Securities Act or any state securities or blue sky laws,
(c) understands that he must bear the economic risk of said investment for an
indefinite period of time until the shares are registered under the Securities
Act and applicable state securities or blue sky laws or an exemption from such
registration is available, and (d) will not solicit any offer to sell or sell
all or any portion of the shares other than pursuant to an opinion of counsel
reasonably satisfactory to the Company.

          6.   Transfer Restrictions. This Warrant shall be exercisable only by
Holder and shall not be assignable or transferable. Any attempted alienation,
assignment, pledge, hypothecation, attachment, execution or similar process,
whether voluntary or involuntary, with respect to all or any part of this
Warrant or any right hereunder, shall be null and void.

          7.   Reservation of Stock, etc. Issuable on Exercise of Warrant. The
Company will at all times reserve and keep available, solely for issuance and
delivery on the exercise of this Warrant, all shares of Common Stock (or Other
Securities) from time to time issuable on the exercise of this Warrant.

          8.   Replacement of Warrant. On receipt of evidence reasonably
satisfactory to the Company of the loss, theft, destruction or mutilation of
this Warrant and, in the case of any such loss, theft or destruction of this
Warrant, on delivery of an indemnity agreement or security satisfactory in form
and amount to the Company or, in the case of any such mutilation, on surrender
and cancellation of this Warrant, the Company at its expense will execute and
deliver, in lieu thereof, a new warrant of like tenor.

          9.   Notices, etc. All notices and other communications hereunder
shall be personally delivered, telecopied or mailed by first class registered or
certified mail, postage prepaid, at such address of facsimile numbers as may
have been furnished to each party by the other in writing.

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COMMON STOCK PURCHASE WARRANT - Page 4     (Dial-Thru International Corporation)
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          10.  Miscellaneous. This Warrant and any term hereof may be changed,
waived, discharged or terminated only by an instrument in writing signed by the
party against which enforcement of such change, waiver, discharge or termination
is sought. This Warrant shall be construed and enforced in accordance with and
governed by the internal laws of the State of Texas. The headings in this
Warrant are for purposes of reference only, and shall not limit or otherwise
affect any of the terms hereof. The invalidity or unenforceability of any
provision hereof shall in no way affect the validity or enforceability of any
other provision.

                           [SIGNATURE PAGE FOLLOWS]

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COMMON STOCK PURCHASE WARRANT - Page 5     (Dial-Thru International Corporation)
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DATED as of December 22, 1999.

                                             DIAL-THRU INTERNATIONAL CORPORATION



                                             By:     /s/ Roger D. Bryant
                                                    -------------------------
                                             Name:    Roger D. Bryant
                                                    -------------------------
                                             Title:   Chairman
                                                    -------------------------

                                             Address:  8100 Jetstar, Suite 100
                                                       Irving, Texas  75063
                                                       Fax: (972) 929-1616


                                             HOLDER:
                                             ------


                                             /s/ Lawrence Vierra
                                             --------------------------------
                                             Printed Name:  Lawrence Vierra


                                              Address:
                                                        ---------------------
                                                        ---------------------
                                                        Fax:
                                                            -----------------


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COMMON STOCK PURCHASE WARRANT - Page 6     (Dial-Thru International Corporation)
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                                   EXHIBIT A

                     FORM OF NOTICE OF EXERCISE - WARRANT
                     ------------------------------------

               (To be executed only upon exercise or conversion
                      of the Warrant in whole or in part)

To Dial-Thru International Corporation

          The undersigned registered holder of the accompanying Warrant hereby
exercises such Warrant or portion thereof for, and purchases thereunder,
______________/12/ shares of Common Stock (as defined in such Warrant) and
herewith makes payment therefor (including any Tax Withholding Liability) of
$__________, as of the date written below. The undersigned requests that the
certificates for such shares of Common Stock be issued in the name of, and
delivered to, _______________________________________________ whose address is
_____________________________________________________________________________.


Dated:
       -----------------------------

                                              ----------------------------------
                                              (Name must conform to name of
                                              Holder as specified on the face of
                                              the Warrant)



                                              By:
                                                     ---------------------------
                                              Name:
                                                     ---------------------------
                                              Title:
                                                     ---------------------------

                                              Address of Holder:

                                              ----------------------------------
                                              ----------------------------------
                                              ----------------------------------


Date of exercise:
                  --------------------------


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/12/   Insert the number of shares of Common Stock as to which the accompanying
Warrant is being exercised. In the case of a partial exercise, a new Warrant or
Warrants will be issued and delivered, representing the unexercised portion of
the accompanying Warrant, to the holder surrendering the same.