I used an unlocked-by-AT&T iPhone 3GS in Honduras (Claro and TIGO), El Salvador (Claro and TIGO), Costa Rica (ICE), and Panama (Claro) for all of 2012 into 2013. The stores all worried when I said iPhone because they didn't have the 4S-sized sim card, but once I said it was a 3GS and a normal sized sim, they were relieved and helpful. It wasn't always unproblematic. In Honduras (Roatan and LaCeiba) and El Salvador (San Salvador) I had to see several people in stores to find the person who could figure out how to get the data to work. The sim card didn't quite know how to see the data plans. I tried to figure out what they were setting so I could learn and write it up for others but they didn't seem to know what they were doing.

So phone calls will be simple. Be patient with the data part and test it before leaving the store. And go to the company stores to buy the sim and get started. After that you can recharge both voice and data anywhere or with the pre-paid code cards and use texting to renew.

As far as how to unlock your phone, let Apple or AT&T tell you as that may change.