I looked into Asterisk a while ago. It has grown rapidly in past few years as more people find out about it. The one nice thing about it is Cisco lets you download the required files to update their phones to work with Asterisk for free.
I didn't know about that feature. I implemented Evolution PBX for one of our subsidiary companies. Evolution PBX runs on Asterisk. We ordered some older phones that would work with Asterisk, I believe they were Avaya VoIP phones. I wish I would of known that Cisco phones worked with Asterisk, we have a bunch of Cisco 7912 phones laying around.