T-Mobile has a VoIP service that has been in use on Facebook for a bit now. Bobsled has been extended to cover more platforms with the service on the iPhone and more important available on Android now as well. The service is used via a free app that you can download and the app allows calls to me made to Facebook contacts at this point.
The service does support free landline calls to the US, Canada and Puerto Rico via a browser apparently. To make those free browser calls you need to log in through the browser interface. The Android app will allow you to make Bobsled-to-Bobsled calls only. The good news is that with Android and iOS apps most of the people you know are probably callable using bobsled.
You can leave voice mails and the voice mails are sent to the contacts Facebook wall. You can record a voice message as well that will be penned to the wall. I wonder if that means everyone can read or hear the message you leave. If you want free calls, this will do the trick. The Android app can be grabbed on the Android Market now.