Yeah, as Bruno says, we used to run a asterisk server setup (talk.fedoraproject.org), but it was used about 1-3 times a month, which really didn't make it worth maintaining.
Something thats used more and has less maint costs might be possible.
Have you folks looked at sipwhich?
https://fedoraproject.org/wiki/Features/SIP_Witch_Domain_Telephony http://www.gnu.org/software/sipwitch/
My understanding is that this would allow for a decentralized setup for peer to peer sip stuff, but I could be missing something.
kevin