Originally Posted by
FliesWay2Much
... make sure you have an unlocked quad-frequency GSM phone. AT&T and T-Mobile use GSM in the U.S. Verizon and Sprint do not, so, their phones won't work.
Verizon offers plenty of phones that are quad-frequency, and I assume Sprint does, also. But make sure you are choosing a phone that has a radio frequency compatible with the places you travel.
When out of the US, I recommend using a data SIM and using the combination of Google Voice (get a number local to you for free) and Google Hangouts. Seamless way to dial any US number using essentially VOIP on a cell phone, usually for no charge.