Mexico has just published a couple of standards and I believe that there is a Digital Interface standard amongst them. At one time, TELMEX was busily building an digital business overlay system that was T1/muLaw based. I don't know whether that has continued. So you had the European based (mostly Ericsson) analog system overlayed by a US digital system for the banks and large corporation in the major cities. Than the bottom dropped out of the peso, and frankly I don't know where things stand now but believe that NIST can provide you with copies of NOM-150 and NOM-151. They should tell you what the requirements are. So can NYCE, the Mexican certification organization for Telecom equipment.
Ciao, Vic
