[Bigsmith]

The problem with both Movistar and Telcel is that they offer good rates for calls originated within the local area where you buy/activate the SIM, but rates tend to be significantly higher outside that zone. And it's difficult to get accurate current info on exactly what the rates are in various areas.

For prepaid voice service in Mexico with minimum hassle, I suggest getting an AT&T GoPhone account (set it up before leaving the U.S.) GoPhone's Mexico roaming rate is 25 cents/minute everywhere, INCLUDING calls to the U.S. Coverage within Mexico is pretty good, but I don't think they offer data.