Another option: You can unock many GSM phones by paying for the code and unlocking the phone your self. Do it online:
http://www.unlockmyphonenow.com/ There are many offers for unlocking and some are cheaper than the one I listed above.
Then buy a SIM card for Belize, online or in Belize, that has free incoming calls. I guess Digicell is the the only cell service and incoming calls are free on Pre-paid SIMS. Everyone is just reselling Digicell.
http://www.telestial.com/view_product.php?PRODUCT_ID=LSIM-BZ01& Then sign up for a Call Back service at around .30 minute to call the US from Belize. To use a call back service. You call the service number, hang up, it calls you back and then you input your number for your call to USA or wherever. You incur no airtime charges since your incoming calls are free.
One Call Back service:
http://www.globaltel.com/Rates.html Disclaimer: I have not used this system myself but it is commonly used by others.