When someone says VoIP and Linux, the first thing that comes to mind is Asterisk, the open source PBX software. This is the same software that we deployed for our university several years ago (disclaimer: I am no longer in charge of the service so whatever is happening there is no longer my concern). However, this is not the only open source SIP-compliant PBX software available. The other competitor is SipX by SIPFoundry.
SIPX is the basis of the commercial software that Pingtel distributes. So if you wish to have full commercial-grade support, contact Pingtel. Otherwise, check out the installation guide to get you started. SIPX runs on RedHat, Fedora, CentOS, OpenSUSE, Debian, , Ubuntu, Mac OS X, Windows Server and OpenSolaris. If you are not yet ready to mess up your partition, check out their VMWare appliance as well.
What are you waiting for? Start peering with other SIP servers.