You have a Cingular phone. It is a GSM phone with either 2,3, or all 4 bands. No matter which you have, it will most assuredly have the 1900 MGHZ spectrum used in Belize. So, you have three choices:
1. Google "unlock cell phone (your location)", take it in and have it unlocked (pefectly legal) for about $20, and bring it with you and simply buy a SIM card and prepaid minutes.
2. Pay Cingular's price of $3.05.
3. The easiest, unless you need immediate access to a phone, is to walk into one of three internet cafes and pay $.50 a minute to the USA.