You didn't say:
How long will you need it?
Do you need to make (lots) international calls or US demestic calls only?
How much will be your usage (minutes-wise)?

Depending on your answers, you may get different recommendations from this board. Some may recommend you a unlocked 3G phone from ebay or something. You can get cheap pre-paid cell phones in places like Walmart, Target...etc. IF it's going to be a short stay and light usage, you may even consider getting a Tracfone (available in Walmart, Target, CVS, Walgreens, Office Depot..etc).

The following thread may get your started:

http://www.flyertalk.com/forum/trave...ntl-calls.html