T-Mobile has launched a new Facebook App Bobsled which allows you to make a VOIP calls to your Facebook friends who are also using Bobsled for FREE. All you need to do is add the Bobsled app to your Facebook profile and you are good to go.
According to T-Mobile,
“In the near future, T-Mobile plans to evolve Bobsled by T-Mobile to include video chat, to create the ability to place VoIP calls to mobile and landline U.S. numbers and to offer applications on smartphones and tablets across various mobile platforms, regardless of the carrier that powers such devices.”
More info on Bobsled: http://www.letsbobsled.com