Voice over Internet Protocol (VoIP) calls using applications such as Truphone and Fring are nothing new to the iPhone. VoIP calls using 3G, or EDGE/GPRS for that matter, have been available for three months now, and it's the jailbreak app VoIPover3G that makes them possible.
The app works by using a MobileSubstrate library that tricks applications into behaving as if they are on a Wi-Fi connection when they are actually on a cellular data connection. Fring and Truphone are tricked by default when VoIPover3G is installed using the Cydia package manager.
In order for VoIPover3G to work properly with Intelliscreen installed, Intelliscreen's Performance Boost must be disabled.
VoIPover3G will not install an icon, so users looking to add to the list of tricked apps will need to modify the file /Library/MobileSubstrate/DynamicLibraries/VoIPover3G.plist. In fact, to trick all apps the VoIPover3G.plist file can be removed or renamed. Users have reported decent call quality when using 3G and EDGE data connections. Installing VoIPover3G also allows apps over 10MB to be downloaded regardless of Wi-Fi availability.
Apple has restricted the iPhone SDK to prevent VoIP calls over cellular data connections, presumably to protect carrier revenue. Remember, the installation and use of VoIPover3G is at your own risk!