This video shows you how to use Skype on your iPhone 3G or 3GS. As you know Skype (from app store) can only be used on wifi. This video shows you how to use 3G Unrestrictor (from cydia) so that you can make calls using Skype on 3G. Combine this with Backgrounder (cydia app) and you can now make and receive calls anytime and anywhere.
Add repo.sinfuliphone.com to cydia sources.
Then search for 3G Unrestrictor and install.
(Open 3G Unrestrictor and add Skype to the list.
If 3G Unrestrictor isn't working then try installing VoIPover3G from cydia.
Download Backgrounder from Cydia. Once installed, to enable, open Skype (or any app) then hold the home button, let go of the home button when you see "backgrounder enabled" Now Skype (or any app) will be running in the background, enabling you to receive calls even if your iPhone is not on.
To disable Background it's the same way, hold home button until you see "backgrounder disabled"