For some apps (example Fring) you have to be on Wifi network in order to perform certain functionality. For example, with Fring you cannot make VOIP calls if you are on 3G network.
VOiPover3G app from Cydia for jailbroken phones will allow you to use Wifi only functionality with any connection type – 3G/EDGE/GPRS.
VOiPover3G hack tricks iPhone to think that it is on Wifi while it is on 3G/EDGE/GPRS.
Bottom line, with Fring you can make VOIP calls while on the road with 3G/EDGE/GPRS connection.
Have fun saving on those international minutes.