I would purchase a phone from AT&T like here: (8$):
htt p://w w w.wireless.att.com/cell-phone-service/cell-phone-details/?device=Nokia+2330+-+GoPhone%C2%AE+-+Prepaid&q_sku=sku4830288 (remove the spaces at the beginning)
Once you get it, you can call AT&T and they will give you the unlock code (so you can put in a foreign sim card).
After you land, they have little kiosks by the airport for digicell (and another popular carrier) that you can buy a cheap prepaid sim and pop it into your phone.
Did it last time I was there and still have plenty of time left after calling the US for a good amount of time.