You don't say what rates ATT&t and Sprint are charging you.

Try looking at this site www.mychitchat.com The rates to Mexico look to be about 7 cents a min.

We use this to make international calls to the UK from our cell phones. The rates are the same (just remember that you might be using up your cell phone minutes as well) whether from a land line or cell. Also remember that if you call an international cell phone number there may be extra charges. In the UK most cell phone have "free incoming calls" which actually means the caller pays for calling that number.

Check out the Mexico charges. I know that ChitChat has been really good and economical for calling the UK.