Found this announcement on LWN today, might be worth keeping an eye on it:
GNU Free Call Announced
GNU Free Call is a new project to develop and deploy secure self-organized
communication services worldwide for private use and for public
administration. We use the open standard SIP protocol and GNU SIP Witch to
create secured peer-to-peer mesh calling networks, and we welcome all
participation in our effort. [...]
Will use Gnupg for authentication.