PM in Mexico is only good for Voicemail (which you'll have to dial from another number) and for sending/receiving Texts (without attachments, because no Data, unless you can access WiFi).
If you want outgoing/incoming Talk or mobile Data then think the best solution is a dual-SIM phone with a local cellular subscription. Mexico basically has lots of little carriers and three big ones: AT&T Mexico (not the same company as AT&T USA), Telcel (most coverage and highest prices, best choice if you'll travel lots of Mexico), MoviStar (the one I chose because locals said it was the best service in the Baja area).
You can get all sorts of travel SIMs with all sorts of international rates and coverages. I've never found any (of the bewildering many) which could offer better rates than local carriers when travelling Canada, USA, Mexico, Germany, and Poland. In fact, they're usually (in my opinion) a complete rip-off, but I guess they charge so much extra to buy lazy worry-free convenience.
