Fring :

Fring is a free software that allows calls to mobile. For this, it uses your mobile internet connection: WiFi, GPRS or 3G.
Moreover, fring is compatible with most instant messaging protocols like MSN Messenger, Google Talk, Aim, Skype. You can also chat with your usual contacts.

How does it work? :
1- Login fring.com and register your phone model and your number.
2 -You will receive an SMS with an Internet link to download the software.
3 -Once it is installed, just configure each service and possibly Wi-Fi the most commonly used. In the next starts, fring will use first the Wi-Fi around. If it is out of range, then use cellular connections.
- Here is a short video presentation using the service :