Are you doubtful that it is possible?
Well, you don’t need to be an hacker or a computer geek, but simply you need a cellular phone, a free Wi-Fi network and an application called fring.
In the video below is shown how this is possible thanks to fring, an application that uses your mobile internet connection to make peer-to-peer VoIP calls.
By using a Wi-Fi network connected to internet it isn’t even necessary a sim card!
A good idea to use your home or office wireless network for an unusual way to make and receive calls with your mobile device.
Furthermore you can use this system also in airports, hotels and all places that offer you a free hotspot.
Oh, I nearly forgot that fring works over 3G, GPRS and Wi-Fi networks and dynamically adapts itself to the optimal network !