Exhibit 4.2

TWELFTH AMENDMENT TO

MANAGEMENT SERVICES AGREEMENT

Between

AT&T MEXICO, LLC

(FORMERLY KNOWN AS AT&T MEXICO, INC.)

A limited liability company duly formed under
the laws of the State of Delaware, United States of America, with headquarters in Wilmington, Delaware, USA, hereinafter AT&T MEXICO, with Permanent Establishment in the United Mexican States. Establishment in the terms provided in
the Income Tax Law with address at Lago Zurich No. 245, Edificio Presa Falcón, Piso 14, Colonia Ampliación Granada, 11529, México, D.F.

And

AMÉRICA MÓVIL, S.A.B. DE C.V.

A sociedad anónima bursátil de capital variable duly organized under the laws of the United Mexican States, with its principal place
of business in Mexico City, D.F., hereinafter AMÉRICA MÓVIL.



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This TWELFTH AMENDMENT TO MANAGEMENT SERVICES AGREEMENT is made as of July 12, 2012
(the Amendment), between AT&T MEXICO and AMÉRICA MÓVIL.

WHEREAS, AT&T MEXICO and
AMÉRICA MÓVIL entered into that certain Management Services Agreement dated February 27, 2002 as amended by certain First Amendment to Management Services Agreement dated as of January 1, 2003; that certain Second Amendment
to Management Services Agreement dated as of October 29, 2003; that certain Third Amendment to Management Services Agreement dated as January 1, 2004; that certain Fourth Amendment to Management Services Agreement dated as of
January 31, 2005; that certain Fifth Amendment to Management Services Agreement dated as of December 31, 2005; that certain Sixth Amendment to Management Services Agreement dated as of February 1, 2007; that certain Seventh Amendment
to Management Services Agreement dated as of July 7, 2010; that certain Eighth Amendment to Management Services Agreement dated as of September 23, 2009, that certain Ninth Amendment to Management Services Agreement dated as of
November 18, 2009; that certain Tenth Amendment to Management Services Agreement dated as of August 3, 2010; and that certain Eleventh Amendment to Management Services Agreement dated as of February 23, 2011 (as amended, the
MSA).

WHEREAS, AT&T MEXICO and AMÉRICA MÓVIL desire to amend the MSA as provided herein.

NOW, THEREFORE, in consideration of the premises and the mutual agreements and covenants hereinafter set forth, the parties
hereto hereby agree as follows:

SECTION 1. The parties agree that the first sentence of clause Sixth of the MSA is hereby
amended to read as follows: This agreement shall be in effect during the period starting on January 1, 2001 and ending on December 31, 2016.

SECTION 2. In accordance with the terms of Clause FOURTH of the MSA, AMÉRICA MÓVIL shall pay AT&T MEXICO TEN MILLION US DOLLARS (US $10,000,000.00) plus value added tax, for the eleventh
year (2012) of the term of the MSA in compensation for services rendered by AT&T MEXICO thereunder. AMÉRICA MÓVIL shall make this TEN MILLION US DOLLAR (US $10,000,000.00) payment to AT&T MEXICO in twelve monthly
installments of EIGHT HUNDRED THIRTY THREE THOUSAND THREE HUNDRED THIRTY THREE US DOLLARS and THIRTY THREE CENTS (US $833,333.33), plus value added tax, through wire transfer of immediately available funds payable in United States Dollars to a bank
account designated by AT&T MEXICO (which monthly payments commenced in January 2012). No later than the end of November 2012, the parties shall begin consultations and use best efforts to agree on compensation to be paid to AT&T MEXICO for
the remaining term of the MSA.



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SECTION 3. This Amendment and the MSA are hereby confirmed as being in full force and
effect.

SECTION 4. This Amendment may be executed in one or more counterparts, and by the different parties hereto in
separate counterparts, each of which when executed shall be deemed to be an original but all of which taken together shall constitute one and the same agreement.

SECTION 5. For interpretation and fulfillment of this Amendment, the parties expressly agree to submit to the laws of the territory and courts of Mexico, City, D.F., waiving the application of any other
law or jurisdiction of any court that might have jurisdiction over them by reason of their current or future address.

This
agreement is entered in Mexico, City, D.F. on the date first set forth above.



|                 |  |                             |  |                               |  |     |  |                                        
|---------------- |  | --------------------------- |  | ----------------------------- |  | --- |  | ---------------------------------------
|AT&T MEXICO, LLC |  |                             |  | AMÉRICA MÓVIL, S.A.B. DE C.V.
|                 |  |                             |  |                              
|By:              |  | /s/ Jeffrey McElfresh       |  |                               |  | By: |  | /s/ Alejandro Cantú Jiménez            
|                 |  | Jeffrey McElfresh
President |  |                               |  |     |  | Alejandro Cantú Jiménez General Counsel