Screenshots from O2 TU Go
Even devices that are not normally associated with phone calls like the Apple iPod or Apple iPad will be able to make or take calls using O2 TU Go and a third party calling application like Skype. Once you've downloaded the app, you simply use an SMS message on your mobile phone for verification and you're off and running. Up to five devices can be connected via the app which means that if you receive an incoming call, all connected devices will ring.
Calls made using the O2 TU Go app will be billed at normal rates. While the app is now available for O2 customers at iTunes and the Google Play Store, O2 says that it will soon be available at BlackBerry World and the Windows Phone Store.
source: GooglePlay via Pocketlint