<PAGE>

                                                                    Exhibit 10.5

MASTER AGREEMENT FOR THE PROVISION OF TELECOMMUNICATION SERVICES EXECUTED
BETWEEN "American TeleSource International, Inc." and BESTEL, S.A. DE C.V.

Master Agreement for the provision of Telecommunication Services executed
between BESTEL, S.A. DE C.V., represented by Lic. F. Xavier Basave Gonzalez and
Ing. Pablo J. Galindo Tovar, ("BESTEL") and "American TeleSource International,
Inc." represented by Charles R. Poole, (the "Customer") in accordance with the
following Recitals and Clauses.

AGREEMENT NUMBER: 100948-0011

The telecommunication Services that BESTEL shall provide to the Customer will be
those shown in the following table and the parties agree that the provision of
such services are governed by the terms and conditions of this Master Agreement
for the Provision of Telecommunication Services and by that established in the
Addenda corresponding to those Services and/or Promotional Programs selected:

<TABLE>
<CAPTION>
--------------------------------------------------------------------------------------------------------------
SERVICES AND ADDENDA OF                CONTRACT DATE                          CUSTOMER'S SIGNATURE
THIS PRESENT AGREEMENT
--------------------------------------------------------------------------------------------------------------
<S>                                    <C>                                    <C>
LONG DISTANCE                          xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx        xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
                                       xx                                     x
--------------------------------------------------------------------------------------------------------------
OPERATOR SERVICES                      xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx        xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
                                       xx                                     x
--------------------------------------------------------------------------------------------------------------
800 SERVICES                           29/SEPT/98

--------------------------------------------------------------------------------------------------------------
PRIVATE LINE SERVICES                  xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx        xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
                                       xx                                     x
--------------------------------------------------------------------------------------------------------------
DATA SERVICES                          xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx        xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
                                       xx                                     x
--------------------------------------------------------------------------------------------------------------
LIT FIBER                              xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx        xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
                                       xx                                     x
--------------------------------------------------------------------------------------------------------------
DARK FIBER                             xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx        xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
                                       xx                                     x
--------------------------------------------------------------------------------------------------------------
COLLOCATION                            xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx        xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
                                       xx                                     x
--------------------------------------------------------------------------------------------------------------
INTERNET SERVICES                      xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx        xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
                                       xx                                     x
--------------------------------------------------------------------------------------------------------------
OTHER SERVICES (SPECIFY)               xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx        xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
                                       xx                                     x
--------------------------------------------------------------------------------------------------------------
</TABLE>


                                  CONDITIONS

THE CUSTOMER accepts and acknowledges that once the Addenda corresponding to the
Services and/or Promotional Programs selected are executed, these shall form an
integral part of this Master Agreement for the Provision of Telecommunication
Services.

The Addenda corresponding to the Services and/or Promotional Programs selected
at a later date than the date of execution of this Master Agreement, shall have
an effective date as of the date of execution and acceptance and shall become an
integral part of this Agreement.

                                   RECITALS

I.   BESTEL states through its legal representatives:

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<PAGE>

That they represent a Corporation duly incorporated under the laws of Mexico
under the name of "Cableado y Sistemas" as is shown in Corporate Charter Number
33,515 dated August 1, 1995 and granted before Notary Public number 70 of
Guadalajara, Jalisco and registered in the Public Commerce Registry of that city
under number 177-178, Volume 586, First Book.

That on September 2, 1996 its corporate name changed to "Bestel", S.A. de C.V.
as is shown in corporate charter number 26,414 before Notary Public number 42 of
Guadalajara and registered in the Public Commerce Registry of that city under
number 146 of Volume 613, First Book.

That the personality and capacity of its legal representatives as of this date
has not been modified nor limited in any way.

That on January 8, 1996 the Federal Government through the Ministry of
Communications and Transportation (the "Ministry") granted a concession  to
install, operate and exploit a Public Telecommunications Network and therefore
has the ability to provide the Services under this Agreement.

That it desires to provide the Services under this Agreement to the Customer.

That its tax identification number is BES950801CA1 and its address is Lopez
Cotilla #1987-A, Colonia Obrera Centro, Guadalajara, Jalisco, 44140.

That it has the personnel and technical capacity, as well as its own
infrastructure or that of a third party necessary to provide the Services under
this Agreement.

II.  THE CUSTOMER states through its legal representative:

That it represents a Corporation duly incorporated under the laws of the State
of Texas under the name of "American TeleSource International, Inc.".

That the personality and capacity of its legal representative as of this date
has not been modified nor limited in any way.

That its address is 12500 Network Boulevard, Suite 407, San Antonio, TX 78249.

That its telecommunications terminal equipment to be utilized by the CUSTOMER is
duly homologated before the competent authorities and can be interconnected to a
public telecommunications network without causing interferences or any damages
to said network and complies with the authorized signaling.

That it desires to contract with BESTEL the Services shown on the First Page of
this Agreement which after its execution forms an integral part hereof.

That it desires to commit to the traffic corresponding to the Services under
this Agreement in the percentages shown hereunder or in its Addenda.

Based on the above, the parties agree to the following:

                                    CLAUSES

1.   OBJECTIVE.

 .1   BESTEL shall provide to the CUSTOMER the Services shown on the First page
     of this Agreement (the "Services") and THE CUSTOMER agrees to pay the
     consideration for such Services as established in the corresponding
     Addenda.

 .1   In the event THE CUSTOMER receives the Services or opts for any of the
     Promotional Programs

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<PAGE>

     (modified from time to time) it is understood that THE CUSTOMER has
     accepted the applicable terms and conditions for those Services and/or
     Promotional Programs registered with the Federal Telecommunications
     Commission.

2.   CONSIDERATION.

2.1  THE CUSTOMER agrees to pay BESTEL the total value of the consideration
     specified in the corresponding Addenda for the Contracted Services (the
     "Price") by no later than the date agreed upon.

2.2  The Price for the Services agreed to by the parties shall be subject to
     modifications in accordance with the changes that could arise under the
     terms and conditions of the Promotional Program under which the Customer
     has subscribed.   In the event of conflict between the terms and conditions
     of this Agreement and those of the Promotional Program selected by the
     CUSTOMER, those contained in the Promotional Program shall prevail.

2.3  Except for that established in the Addenda, the rates can be modified at
     any time by BESTEL. The CUSTOMER acknowledges that BESTEL can make
     adjustments or discounts of such Price as of the effective date for the
     month of such adjustments or discounts.  Such adjustments or discounts
     shall be stated in BESTEL's invoices presented to the CUSTOMER on the
     effective date of such rates.  BESTEL shall be able to apply any
     adjustments or discounts against any outstanding amounts owed by the
     CUSTOMER.

2.4  All payments specified in this Agreement shall be made in accordance with
     that established in the corresponding service Addendum.

     In the event that the Price or any consideration would be established in
     another currency that is not Mexico's legal currency, but legal in any
     other country, the CUSTOMER agrees to pay BESTEL in such preestablished
     currency or in Mexican pesos at the exchange rate published by the Bank of
     Mexico in the Federal Daily Gazette on the date payment is made by the
     CUSTOMER.

2.5  The CUSTOMER acknowledges and accepts that if any other carrier would
     charge BESTEL any other amount  for access to BESTEL's network required by
     the CUSTOMER or its customers, the CUSTOMER shall reimburse BESTEL such
     amounts.

2.   FORM AND PLACE OF PAYMENT.

3.1  BESTEL shall send on a monthly basis an invoice to the CUSTOMER  for the
     Services provided to the address shown in this Agreement.

3.2  Payment for the invoice(s) for the Services provided can be made in cash at
     the locations provided by BESTEL on such invoices or by check payable to
     BESTEL, S.A. DE C.V., by wire transfer or any other form specified by
     BESTEL.

3.3  Any differences that the CUSTOMER should have with the charges billed on
     the invoice should be directed to the Customer Service Center established
     by BESTEL. In the event the CUSTOMER wishes to dispute, CUSTOMER shall do
     so in writing by no later than the payment date shown on the invoice.

3.4  The invoices should be received by the CUSTOMER within the corresponding
     delivery time set for such CUSTOMER and can be modified by BESTEL from time
     to time.  If the CUSTOMER does not receive the invoice, the CUSTOMER shall
     notify BESTEL so that a copy can be sent.  Notwithstanding the above, the
     CUSTOMER shall pay immediately any outstanding amounts.

3.5  The CUSTOMER shall pay to BESTEL the amounts specified on the corresponding
     invoice by no later than the date established on the invoice which shall
     always be twenty (20) days after the close of the billing period.  The
     CUSTOMER shall abide by the established in Paragraph 2.4 above.

                                      26
<PAGE>

3.6  THE CUSTOMER agrees that BESTEL may apply the payment for one of more of
     the Services under this Agreement to any outstanding amount generated by
     the provision of any of the Services for an amount or in any invoice order
     established by BESTEL.

3.7  BESTEL shall be able to partially or totally suspend the provision of the
     Services under this Agreement upon previous notice to the CUSTOMER, in the
     event the CUSTOMER pays late or does not completely pay its outstanding
     amounts.  In the event of suspension and in order to reestablish the
     Services, the CUSTOMER shall pay any outstanding amounts as well as pay any
     reconnection charges.  As well, in the event BESTEL so determines, the
     CUSTOMER shall provide BESTEL any security requested.

3.8  The amounts owed by the CUSTOMER after the payment date of the invoice
     shall be subject to late payment fees calculated by multiplying the TIIE
     Rate ( Interbank Interest Rate) published in the Federal Daily Gazette
     effective on the date of payment or substituted rate times 2.0 for the
     entire late period and until total payment has been made.

3.9  BESTEL shall be able to request total payment of any amount owed by the
     CUSTOMER before continuing to provide the Services under this Agreement.
     In the event that BESTEL should omit in any invoice amounts referred to in
     this Paragraph, such charges can be invoiced in any subsequent invoice in
     order for the CUSTOMER to cover its outstanding amounts.  Such omission
     does not constitute a waiver by BESTEL to collect any amounts nor shall it
     be interpreted as the CUSTOMER'S right to not pay such amounts.

4.0  TERM.

4.1  This Agreement shall be in force as long as any Service in accordance with
     the corresponding Addenda remains in force unless otherwise specified.

4.2  In the event the CUSTOMER stops receiving BESTEL's services for any cause
     and receives them from another vendor or has terminated this Agreement, the
     parties agree that in the event the CUSTOMER desires to receive the
     SERVICES again and receive the Promotional Programs, it shall be understood
     that the CUSTOMER has accepted the terms and conditions established
     hereunder, which will be in force under the specified in subparagraph 4.1
     above.

                                      27
<PAGE>

5.   TERMINATION.

5.1  In the event the CUSTOMER does not fulfill any of its obligations under
     this Agreement, BESTEL shall terminate this Agreement without any
     liability.  If this occurs, all amounts owed by the CUSTOMER to BESTEL at
     that time shall become due immediately.

5.2  BESTEL shall notify the CUSTOMER the termination of this Agreement
     indicating the amount owed and the due date in which such amount is to be
     paid.  Those amounts owed by the CUSTOMER as of the termination date shall
     cause late fees as mentioned in 3.8 above.

5.3  In the event of default by BESTEL, the CUSTOMER shall notify BESTEL the
     cause(s) of such default so that BESTEL may correct to the CUSTOMER's
     satisfaction.  After thirty (30) days and if such default continues, the
     CUSTOMER shall be able to terminate this Agreement, after paying any
     amounts owed to BESTEL without any liability.

5.   SERVICE INTERRUPTIONS.

6.1  BESTEL shall not be responsible for the suspension or interruption of the
     Services because of force majeure or fortuitous cause or because of
     unforeseen circumstances including transmission failures as well as the
     suspension or interruption of communication by other networks through which
     the signals or traffic runs through.

6.2  BESTEL shall be able to interrupt for any necessary time the provision of
     the Services under this Agreement when an inspection or maintenance is
     needed for their facilities and/or equipment.  BESTEL shall try to program
     such inspection or maintenance at hours that do not result inconvenient for
     the CUSTOMER.

6.3  Unless otherwise specified in this Agreement, the CUSTOMER acknowledges
     that the equipment and the lines through which the switched or dedicated
     local service is to be provided are not owned by BESTEL; therefore BESTEL
     shall not be responsible for any failures attributable to such equipment or
     lines.  BESTEL shall only be responsible for the Services provided through
     circuits, equipment and fiber optic network owned by BESTEL or affiliates.

6.4  Unless otherwise specified in this Agreement, neither party shall be
     responsible for any damages, including indirect and/or consequential or for
     the loss of income derived or related with the provision of the Services
     under this Agreement.

5.   LIABILITY.

7.1  According to the type of Services provided, the CUSTOMER shall not use such
     Services for international callback that utilizes the signaling of an
     incomplete call to any country where this type of service is not legal.  As
     well, the CUSTOMER shall not use the contracted Services for transport of
     switched voice services to or from any other country since this is a
     violation of Mexican law.  The CUSTOMER hereby assumes such responsibility
     and shall be responsible for any claims arising against BESTEL .

7.2  The CUSTOMER acknowledges that the Services provided hereunder and
     especially those of the corresponding Addenda cannot be marketed by the
     CUSTOMER unless the CUSTOMER has the licenses and/or authorizations to do
     so.

                                      28
<PAGE>

7.3  The CUSTOMER shall indemnify and hold BESTEL harmless from any damages,
     costs, responsibility and expenses resulting from such connections or
     connection intents and marketing of Services including those damages
     resulting from the use or unauthorized access to BESTEL's Public
     Telecommunications Network.

7.4  If necessary, the CUSTOMER hereby authorizes the Federal Telecommunications
     Commission, the Auditor of the Long Distance Carriers Committee and BESTEL
     to supervise the international private lines contracted through the
     execution of the corresponding Addenda authorizing BESTEL to immediately
     terminate this Agreement without any liability in the event misuse is
     detected.

7.2  BESTEL shall not be responsible for the contents of the information that
     the CUSTOMER transmits through its telecommunications network.

5.   RELATIONSHIP OF PARTIES.

8.1  The nature of this Agreement is essentially commercial since BESTEL and THE
     CUSTOMER are companies of everyday business activities and in addition have
     the necessary elements to fulfill each and every obligation with their
     employees.  As well, between the CUSTOMER' s employees and BESTEL's
     employees there does not exist any relationship whatsoever; therefore there
     is no labor relationship.  THE CUSTOMER shall be the only party responsible
     of the obligations that the law establishes as an employer with regards to
     the personnel utilized and shall be responsible for each and every
     individual and collective claim that the CUSTOMER's employees could present
     against BESTEL assuming the liability of any claim presented and reimburse
     immediately any legal expense or of any other nature that BESTEL could
     incur for such concept.  In the same fashion, BESTEL shall be the only
     responsible party of the obligations that the law establishes as an
     employer with regards to the personnel utilized and shall be responsible
     for each and every individual and collective claim that BESTEL's employees
     could present against the CUSTOMER assuming the liability of any claim
     presented.

5.   TROUBLE REPORTS/REPAIRS

9.1  The CUSTOMER shall notify its trouble reports to BESTEL and BESTEL shall
     make available to the CUSTOMER a Customer Service Center that will be
     available 24 hours a day, 365 days per year where trouble tickets will be
     reported.  BESTEL shall assign a confirmation number.

5.   SERVICE GUARANTEES.

10.1 The guarantees offered by BESTEL are described and detailed in the
     Promotional Programs selected by the Customer and shall be attached to this
     Agreement forming an integral part hereof.

5.   CREDIT INVESTIGATION.

11.1 THE CUSTOMER states that the information provided regarding its solvency
     and payment capacity is correct and based on that information BESTEL has
     made the decision to provide the Services under this Agreement.  THE
     CUSTOMER hereby authorizes BESTEL to verify such information.  As well, the
     CUSTOMER agrees that this authorization shall continue in force during the
     term of this Agreement for BESTEL's benefit.

5.   NOTICES.

12.1 The parties agree that all notices, communications and notifications
     regarding this Agreement shall be directed to the addresses mentioned in
     the recitals of this Agreement.  Such notices shall be made in writing

                                      29
<PAGE>

     through certified mail or courier with a return receipt, facsimile or any
     other method that the other party has received such notification.  In the
     event of address change, the parties agree to notify the other party with
     at least fifteen (15) days advance notice; otherwise the latest address
     shall prevail.

5.   ASSIGNMENT; MODIFICATION.

13.1 THE CUSTOMER agrees that the rights and obligations derived under this
     Agreement shall not be assigned, transferred, negotiated nor modified in
     any way without the previous consent in writing from BESTEL.  As well,
     BESTEL may assign its rights and obligations derived under this Agreement
     to any of its subsidiaries or affiliates or guarantee any obligation
     through notification to the CUSTOMER.

13.3 THE CUSTOMER shall notify BESTEL with sixty (60) days advance notice any
     modification to its corporate name, otherwise all documentation shall be
     generated using the previous name.DISPUTE RESOLUTION.

14.1 The parties express their firm conviction that in good faith in the event
     there are differences or disputes because of the interpretation,
     fulfillment and execution of this Agreement and in unlimited form for any
     technical aspect, service provision, application and collection of
     consideration and any other that require specific technical capacity, they
     shall reasonably try to resolve in a friendly fashion through mediation
     and/or conciliation, voluntarily and previous to any other procedure that
     the parties could have and take place during a term of thirty (30) days in
     which the parties promote a mutual consulting process in order to resolve
     or avoid any controversy or dispute without waiving their rights.

14.2 It shall be considered that the attempts to reach a friendly solution have
     failed when one of the parties notifies the other party at the termination
     of such term that the negotiations have not been satisfactory; therefore
     the parties may execute their rights or effect any action as specified in
     this Agreement.

14.3 For anything relating to the Agreement including its interpretation,
     fulfillment and execution of, the parties hereby agree to submit to the
     jurisdiction of the courts in Guadalajara, Jalisco and the Mexican laws
     thereby waiving any other jurisdiction which may correspond to them for
     reason of their present or future domiciles or for any other reason
     whatsoever or the invocation of the protection of their country.

5.   ENTIRE AGREEMENT.

15.1 The parties agree that this Agreement and its Addenda constitute the only
     agreement and agree that this Agreement supersedes any agreement,
     communication, or proposal previously related with the objective of this
     Agreement.

The parties hereby agree and acknowledge that this Agreement is subject to the
terms and conditions of any applicable regulations.  In the event such
regulations are not followed, the responsible party shall cure such failure
without terminating this Agreement unless such cure is not done in a reasonable
time.

The parties execute this Agreement in duplicate in Mexico, D.F. on September 29,
1998.

`THE CUSTOMER'                               "BESTEL"
"American TeleSource International, Inc."    BESTEL, S.A. DE C.V.
(Signature)                                  (Signature)
By: Charles R. Poole                         By: Lic. F. Xavier Basave Gonzalez
Title: President                                 Pablo J. Galindo Tovar
                                             Title: Legal Representatives

                                      30