One of the limitations of VOiP iPhone apps such as Fring and Truphone that are available on the App Store is that you can make VOiP calls only over Wi-Fi and not over the your iPhone's data connection. This is due to limitation of Apple's iPhone SDK which was most likely enforced on Apple by carriers.
But there is good news, iPhone Hacker, crash-x, has developed a clever little hack aptly called VOiPover3G that tricks iPhone apps into thinking that they are on Wi-Fi even though they are on 3G/EDGE/GPRS. Thus allowing you to make VOiP calls over your iPhone's data connection.
VOiPover3G developed by crash-x is a MobileSubstrate library that tricks iPhone apps into thinking that they are on Wi-Fi even though they are on 3G/EDGE/GPRS.
It allows users to make VOiP calls over your iPhone's data connection. You can configure which iPhone apps get tricked by adding the app's bundle identifier to "/Library/MobileSubstrate/DynamicLibraries/VoIPover3G.plist”. By default, VOiP apps such as Fring, Truphone and iPhone apps such as iTunes Store and App Store have been added (however the developer has mentioned that iTunes Store and App Store does not work).
If you want to trick all iPhone apps then you need to rename or delete VoIPover3G.plist file, however it is recommended that you selectively add the iPhone apps rather than tricking all apps.
I tested Fring after installing VOiPover3G over my iPhone's 3G data connection and it worked quite well, the sound quality of the calls were good. I am not sure how well it would work over slower data connections such as EDGE and GPRS but iPhone users have reported that it works fine over EDGE.
You can install VOiPover3G on your jailbroken iPhone via Cydia. An important thing to note is that there is no iPhone app icon available on the iPhone's home screen, however after installing the MobileSubstrate library, you should be able to use apps such as Fring and Truphone over your iPhone's data connection which earlier would have given “You must be connected to WiFi” error.
However, please proceed with caution as making VOiP calls over your data connection could get you into trouble
with your carrier.
VOiPover3G has been around for quite sometime, have you used it before? As always, let us know what you think about the hack and how it goes in the comments below.