While most Open Source projects are applications and utilities intended for single users, David Bryan and David Kelly did something different. They created an infrastructure project — a VoIP phone system that either can run on a single box attached to a couple of IP phones or can scale up to a network of hosts processing hundreds of calls between thousands of users. In this informative technical article at ELJonline, Bryan and Kelly detail the 'Vovida Open Communications Applications Library' ('VOCAL') project, a fully functional phone system that can run on either Red Hat Linux or Sun Solaris.

http://www.linuxdevices.com/articles/AT5348974740.html