(IP Telephony, VoIP and Video Conferencing) Ekiga configuration for LAN with same subnet

I am looking for an Open Source Voice Chat Application, like Skype, but within an isolated intranet (no Internet is available). Is there any application which can run on Scientific Linux or CentOS? Client-Server will be OK, but our need is Client Based. **Update:** As per Anonymous answer I have installed Ekiga. In two of Scientific Linux Terminal. These machines are in one network (same subnet), having 192.168.3.51 and 192.168.3.56. When I open Ekiga, both users are visible online to each other in neighbours section. But when I am trying to make a call, message appears user is not available. And when I try to send a message, in message box, after sending a message this error appears: NOTICE: Could not send message While configuring I choose I do not want to sign up for ekiga.net free service I do not want to sign up for ekiga call out service because Internet is NOT available to any of my Linux boxes. Is there any configuration missing?