Here are the steps that has to be follow in-order to enable the use of Skype over 3G. Please note that as to follow these instructions you must have jailbroken iPhone.
The first step requires you download and install Skype from the App Store and the next step requires you launch Cydia from the springboard.
Now press to choose the Sections tab at the bottom of the screen.
The second step require you to choose the System sections and press the Mobile Substrate from the list of packages.
Well, folks if you don’t have Mobile Substrate installed then press the Install button at the top right of the screen.
Tap the Confirm button in-order to begin installation..
Press to select the System section and tap on the Sections tab at the bottom of the screen.
Press the Home button and launch Skype from your SpringBoard. Select a contact to call from the contacts tab and push the call button to call your contact.
You can now notice that the call is completed over the 3G network!