Uses SIP standard for calls
A beta version
Microsoft Office Communications Server 2007 is out and you can sign up to download it today and it adds VoIP support. You might not have heard about Office Communications Server, but the previous version came out in 2005.
Office Communications Server is a private messenger type service and it has its own client called Office Communicator. It allows for somewhat more secure IM services to be run by corporates and it works largely like MSN Messenger with the advantage of it being run of your own servers.
Office Communications Server supports IM type messages, video conferencing and now VoIP with the 2007 version. Microsoft has based the VoIP service on SIP which means that you can call a wide range of other VoIP services, land lines and mobile phones, but not Skype users.
This should be an appealing package for companies located in different locations across the same country or the world as it's a much more controllable envirmonent than using a public IM service. The downside is that you're limited to those with access to your Office Communications Server, as you can't chat with MSN Messgenger users.
You can find out more here