There are only 2 carriers in Belize, BTL and Smart but only BTL uses simm cards so you will have to go with them. Your phone needs to be unlocked. BTl cards are ($25US) 50BZD and come with a $5 USD ($10bzd) credit. You can then add more as you need.
You can also use Skype to make local calls if you have money in your account and it is far cheaper (free) to call home. The better resorts have free Wifi.
Check out our website www.sanpedrobreeze.com
for more info about Belize.