How to trick your iPhone into thinking its on Wi-Fi
Here are the steps that has to be followed in-order to trick your device into pretending that it is on Wi-Fi and which provides VOIP over 3G.
Please note that you need a jailbroken iPhone to make this tutorial work.
Press the Confirm button at the top right of the screen.
You may now use App Store, Fring, Truphone, and the iTunes Store while on 3G. Your phone will act as though you are on Wi-Fi.