If you are an AT&T GoPhone user on the $45 per month plan, take a look at the Your Plan section of the Rate Plans page in your MyAT&T online account manager. A HowardForums user discovered that AT&T says that his plan now includes unlimited calls to Mexico. That's a bit strange as AT&T's advertising and promotional materials for the $45 plan don't say anything about Mexican calling being included with the $45 plan. But there it is plain as day as highlighted below on the At&T site.
Unfortunately, calling Mexico for free on the $45 plan doesn't work. A $45 plan user tried to make a call to Mexico and got shot down with message saying that they didn't have sufficient funds to make the call. I suspect the unlimited calls to Mexico claim on the AT&T site is copy/paste typo (the $60 GoPhone plan includes unlimited calls to Mexico and Canada). Or maybe AT&T is in the process of adding unlimited Mexico calls to the $45 plan and jumped the gun on updating the MyPlan page.
Currently it looks like $45 plan users need to have a cash balance in their account to call Mexico. GoPhone charges 10¢/minute for calls to Mexican and landlines and 26¢/minute for calls to mobile phones in Mexico so you might want to keep those calls short.