Viber - free calls from iPhone :

Viber is a VoIP (Voice over IP) for the iPhone. This application allows free calls to other iPhone users who have installed the application Viber.
The sound quality is better than a regular call. You can call any user Viber the world for free.
This free application works on both Wi-Fi network in 3G network. It will be available on Android and Backberry. Places of emergency http://www.viber.com