1. It goes without saying that the rest of this guide assumes your iPhone is already jailbroken, if not, there’s an excellent guide on how to do it here
2. Once your iPhone is jailbroken to your heart’s content, proceed into the Cydia app and click on the search icon
3. Inside the search screen, look up “VoIPover3G”
4. Once you’ve found the program, click on it to proceed to the next screen
5. Inside this screen, simply click “Install” on the top right hand corner
6. In the next screen, click on “confirm” in the same place you selected on Install
7. Once you confirm, just let Cydia do what it does best and install the software
8. After it’s done, Cydia will want to reload the Springboard. Go ahead and let it.
9. And you’re done! Now you can use 3G as if it were Wi-Fi