Recently I read this story in the New York Times about how to use your mobile phone overseas for cheap.
Apparently, if you have an American cellphone, you can go to another country and buy a local SIM card to use with your phone. Then you can make inexpensive local calls in that country.
It seems, unfortunately, that you can’t bring a mobile phone to America from, say, New Zealand (or the UK), buy a local SIM card, put it in your cellphone, and get service.
It's all locked up. Even if your existing unlocked mobile phone operates on the same frequency, you need to buy a cellphone from the local (US) carrier.