It is quite unusual to hear of a large organisation taking the plunge and moving from an established platform such as Cisco Callmanager into the world of open source. That's why I was quite interested in the following article:

http://www.networkworld.com/news/2006/091206-von-sam-houston.html

It details how Sam Houston State University are in the process of replacing their Cisco CallManagers and legacy Meridian PBXs with an Asterisk based phone system.