For a charge of about 100 pesos every 30 days, there is a service called "Amigo BIP", which allows an unlimited number of text messages to be sent either via a telcel web-page, or via a telcel operator, to an amigo mobile phone.
https://www.atencionenlinea.telcel.c..._amigobip.html
Telcel: Servicios Adicionales
This service can be activated online within a user's "Mi Telcel" account.