Viber is a iPhone app that helps you to place your international calls for free over WiFi and 3G. (Viber works within the US too, but users don’t get charge by AT&T for iPhone-to-iPhone calls, therefore, there is no incentive here) . The service only requires you and your overseas friends to register with your own iPhone number. And you can use Viber app instead of the built-in phone app on your iPhone to make all calls. The look and feels are the same including your standard contact list.
Normal call are routed to the normal iPhone phone app, but if you call your overseas registered friends, the call is routed over VoIP either over WiFi when available or 3G. Watch the below video for more details.
Viber is available in Apple App Store.