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.
The first step requires you to launch Cydia from your iPhone Springboard and select Sections from the tabs at the bottom of the screen.
Tap to select System from the list of Sections.
Now press to select VoIPover3G from the list of Packages.
Press the Install button at the top right of the screen.
Press the Confirm button at the top right of the screen.
Once installation has completed successfully press the large Return to Cydia button.
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.