Check with your service provider and see if they have a roaming agreement with Belize (BTbloodyL).
US T-Mobile and a few others work fine here. Be aware of the costs though.
...or go to a cell phone store anywhere is the States (or do it yourself on the internet) and have your phone unlocked. This will not affect your existing service. They only lock it (sucks) because of agreements re. marketing support between the providers and the phone manufacturers.