Asterisk..... (Score 1)

Take a look at Asterisk http://www.asterisk.org/. The wiki http://www.voip-info.org/wiki-Asterisk has more useful information. It is a full VoIP softswitch solution. In addition to SIP, H323 and MGCP it also supports the IAX protocol, which was designed to be NAT friendly. You won't be able to run it point-to-point. You will have to run an Asterisk server somewhere in your network, but since you are already running Linux on the desktop, it should be fairly easy to run it on one of them for a small network. Combine that with a dynamic dns service like dyndns.org and voila!