Try this out… I’ve just been using it and it’s utterly smart (the back-end service is provided by the chaps at Vivox):
T-Mobile USA has launched itself onto Facebook, providing free VoIP calls between social networkers though the service is not yet integrated into the operators mobile network.
Bobsled by T-Mobile is a VoIP application integrated with Facebook to provide audio connections between users with a tap of the mouse, along with a voice mail service capable of pasting messages to the receivers wall. Thats far from unique; there are a handful of similar services, but the addition of the T-Mobile brand is a significant development.