If you need to make any phone calls while in Mexico buy an international phone card (like a Latadel card) at any market and use that from your hotel or pay phone. The phone system in Mexico is unregulated and if you use your home calling card (ie Sprint, AT&T, etc) you might end up with a surprisingly hefty phone bill. Making calls from pay phones without a Mexican phone card can cost up to $40 US just to CONNECT the call. It's a totally unregulated system. So beware. If you need to make a call from your hotel room make sure you talk rates with the front desk first. Also make sure you buy the phone card once you're in Mexico. My friend bought one in the States and was unable to use it down there.
I too have experienced being told that my cell would work in Mexico when it didn't and paid unreal prices for calling when it did, so I am extremely happy to share my recent discovery about Mexitel Cellular. You can rent a phone from these folks that works well EVERYWHERE in Mexico (at least where we went), costs very little to rent, and costs almost NOTHING to use. We paid (are you ready for this?): $1 for 20 minute calls to the US (or Canada, I'm told), 50 cents to call other cell phones, and incoming calls are $0.00. [They also have a way you can pay $0.22 per min for your incoming calls from the US and make them free for the caller.) To us this was a BIG deal. Hope this helps someone.
Phone calls are expensive. Sign up for international calling before you leave the states. It costs 5 dollars a month with cingular and then 59 cents per minute while in Cancun. Travel with a cell for emergencies. I have been to Cancun ever year since 1988. Wonderful place, but calling home is EXPENSIVE>
cancun is nice but be very careful about public phones like the ones we have at home they are very expensive i only recommend to use LADATEL those are public phones only use cards sold at any shop do not use other and if you use cell phones only use IUSACELL no other companies enyoy what you do take care