T-Mobile has extended its Bobsled VoIP service, broadening use from Facebook to cover access from the browser, from new Android apps, and with apps for iPhone, iPad and iPod touch. In addition to the new apps, there are also free mobile and landline calls in the US, Canada and Puerto Rico, though the Android/iOS software currently only supports calls direct to Facebook contacts.
If you want free general calls, you'll need to log in through the browser interface instead. Still, using the mobile apps you can make Bobsled to Bobsled calls, cross-platform between Android and iOS, and leave voicemails that get flagged up on your Facebook contacts' walls. It's also possible to record a voice message and leave that pinned to someone's wall, rather than actually talk to them.