Based on the description you want or already have a asterisk VOIP (voice over IP) server and you want to integrate this with some sort of messaging server based on xmpp, the end product being something like an internal skype or google talk for your company or group?
I have set up things like this before and they are not too difficult to manage once set up. There are two options to go with here, if you have a machine you want to use as a server we can set that up but it will mean that you are responsible for backups and making sure none of the hardware fails. The other option is to go with a hosted server like amazon's cloud servers, you would not be responsible for the hardware and they guarantee no failure but will mean that if your building loses internet connectivity then it will also lose phone and chat server access. I would recommend the hosted server if you are not a technical person or if this will be used in more than one location.
If you are talking about using just computer to computer calling then the solution is much simpler, we can just put an xmpp server in with a simple addon that allows computer to computer communication. This again will require a server, preferably hosted.
please let me know if I can clear anything up for you, I don't know how technical you are and I would have no problem explaining anything further.