The Open Source Jxta-Wire project and VtalkG, for making calls over P2P. Backend tech only at the moment, though, as far as I can tell.
‘This project explores the possibility of creating a completely decentralized telephone network. A phone network without a hierarchy of mega-switches and no requirement for fault-tolerant central offices. Ultimately, voP2P can become the basis for a phone network with organic growth properties. One in which the user experience improves as the number of worldwide user increases.’