I've saved a lot by unlocking my US phone, buying a Belizean SIM card, and buying minutes there. On certain days, you can buy double or triple the amount of minutes (triple requires a minimum purchase). Sometimes the phone co sent a text about those days, sometimes I just heard about it from friends or on the radio.
I bought my digicell (BTL) sim card just across the street from the int'l airport before I flew to the island. Convenient, but think I probably should have waited to get a smart card, as their minutes are cheaper.