Who offers the best international phone service in Thailand - AT&T, Verizon, or Sprint?

[Dan version 4.0]

I don't need a "local" phone with a Thai SIM card, I have one of those. But I'm looking for a service that works well in Thailand for people back home in America to be able to contact me while I'm travelling. I understand that they all probably use roaming service. Is one better than the other? And what are you recommendations for a particular model of phone?

Thanks!