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.
Do yourself a favor either rent a phone there or buy a cheap unlocked GSM phone and get a SIM in Belize. Or better yet leave phones completely at home. I managed to get around the world quite well without a phone at all in the 60 and 70's.
