Microsoft Office Live Communication Server (LCS) provides enterprise instant messaging and presence information.
Once such a solution is deployed, it's not a too far step from there to integrate it with the existing PBX system via CTI. The typical scenario would be: I'm working on a problem and need advice from a collegue. I check out the presence information to see who's available. How I could either start a chat via the instant messaging (IM) functionality, or phone him. My IM client can find out the phone number of this collegue by retrieving it from Active Directory. Presence information for external contacts can be integrated with the corporate LCS via connectors. In that case the IM client receives the phone number information from the public IM system. Given that the IM client knows the phone number(s), by sending a simple CTI-Command to the exisiting PBX system the phone call could be established.
Once that is in place, turn the employee's PC into a VoIP endpoint by connecting a telephone receiver or headset to the soundcard, and add some software. Alternatively give him a dedicated VoIP phone connected to the LAN. Finally, add a VoIP gateway that routes VoIP calls between the corporate IM system on the LAN and a public VoIP gateway. This pretty much eliminates the need to have a conventional PBX, doesn't it? And Microsoft LCS, being this VoIP/SIP gateway, could play this replacement role.
Another interesting article: http://www.business2.com/b2/web/articles/0,17863,783501,00.html