It depends on your needs

The biggest cell phone carrier is Verizon.  However the 2nd largest carrier of international calls, I believe is the wholesale company behind www.Pingo.com a virtual calling card provider that does over 20 billion mins of international calls a month.

Check your reception from where you call from often at cellreception.com.  so you can make sure that you can "hear me know...great"  

 Then compare cell phone prices apples to apples and pre-program an internet calling card into your phone. So that you don't get whammed with extra cell phone fees for making international calls outside of your set monthly plan