If you download the Google Voice app on your smart phone, you can put your phone on "Airplane" mode, use a wi-fi connection and make unlimited calls for free. Works for Android, Blackberry, iphone, etc. I used the texting feature while in Mexico but haven't tried to make a call yet, THOUGH I'm assuming it'd work. It's ESSENTIAL that you put your smart phone in Airplane Mode, otherwise you'll get hit with crazy roaming data charges. You should do this either way while in Mexico, for you can still browse the net, access your email, Facebook or whatever on your smartphone when you hit a bar or hotel with wi-fi, all the while not incurring any extra fees.
Download the app (you need a gmail account first) at
http://www.google.com/mobile/voice/