Here's the easiest solution:
If you are traveling to Belize from the States and have a GSM Tri/Quad Band phone (Just see if it has a SIM card), then call your service provider (ATT, T-Moblie, etc...) and tell them you are going to be out of the country for some time and that you need the unlock code for your phone. Most phones (minus the really high tech/smart phones) will ask you for a SIM unlock code when an unrecognized SIM is put in. So just call your provider, insist that you need the unlock code and save the info. When you come to Belize go to BTL across the street from Harmouch's Hardware Center, purchase a SIM for $50, put it in your phone, power it on, and put in the afore mentioned unlock code given by your service provider.
Note: Most service providers will tell you they can't do this, but just be persistent, you own the phone, they own the contract and the U.S. phone number....it's the phone you want to take with you so be persistent and they should give it to you. What they want is for you to travel using your existing U.S. cell plan so they can rip you a new one when you receive your bill. Also, if you do get a Digicell SIM instead of a Smart phone here in San Pedro, you can call the States cheaper by dialing 10-10-199-1-(U.S. area code)-phone number, (example 10-10-199-1-504-555-1234). This is a cheaper way to call the States.
Hope this helps,