If I were you I'd buy either a local Digicell SIM card and use it in your iPhone (trimmed as necessary), or buy a cheap "Smart" phone (CDMA) which will be the cheapest to use in Belize.

I recently read of people who took their smart phones to Europe on holiday from Britain and left data roaming on. When they got back they found they had bills for several times the entire cost of their holiday (many thousands of pounds). They took the UK service provider to Court and it was ruled that they had to pay. I STRONGLY recommend you turn off all services on your phone if you intend to bring the SIM card to Belize, and DO NOT use the card except when in your home country.