Elastix is an Open Source Sofware to establish Unified Communications. Elastix goal is to incorporate all the communication alternatives, available at an enterprise level, into a unique solution.
Details for downloading and installing are available here and an overview of the concept and the software is available here.
Update: apparently this product isn’t very well supported and the latest version doesn’t work on the Pi at all 🙁
I tried installing this a few weeks ago. The latest version doesn’t work for the Pi, you have to load the Feb 2013 version. The support is rubbish, as they don’t reply to forum posts. Shame because it would be great to have this working. I think it was a bit of a publicity stunt.
Awww nuts. 🙁