There are lots of great VOIP applications out there for the iPhone, such as iCall which allows you to make free calls to the United States and Canada, Skype and Fring. What was one of the main disadvantages of these applications is that we were unable to use them over our 3G connection, thus only permitting us to make calls over WiFi.
Apple along with AT&T have decided to ban VOIP over 3G connection, thus removing the capability from the iPhone SDK, that only allowed developers to use WiFi connection. However ...