How to use iPhone VOIP over 3G
One of the more disruptive technologies to come along in the mobile space has been the ability to easily make VOIP calls from your handset. Several of the Nokia series handsets (E71, E65, etc) have a SIP client built right into the loaded OS which let you decide whether you want a VOIP call or regular mobile call whenever you dial. The iPhone however has no native SIP client and only recently has VOIP legitimately come to the iPhone in the form of Truphone, Fring and others.
However, although Truphone and Fring will let you make VOIP calls (Truphone through their service and Fring through Skype-out but also an SIP option) Apple has blocked any App Store application from doing VOIP over 3G. So you are stuck with using wifi only.
If you've done a jailbreak on your iPhone you are now in luck though. An app called VOIPover3G will trick Truphone and Fring into thinking they are on wifi even when you are on 3G thus letting you make VOIP calls when you are away from an access point. You can find the app in Cydia by searching for "voip".