Telcel (América Móvil) looks like the only game in town...

TelCel has good coverage everywhere. You can just buy a SIM card for 29p and put it in any phone that has a changeable card, or buy one of their phones. They sell plans, but we just put 100p on our phone every 30 days. The money doesn't expire if you don't use it up, but you have to put money on every 30 days to keep it going. I think the minimum for 30 days is 100p.

End result, it's REALLY inexpensive and easy to charge your phone up anywhere.