The folks at Microsoft are very much into VoIP and have been for many years. SIP has been part of the Windows XP OS since it was released and those who knew how to do it were able to hack Windows Messenger and make it a SIP end point.
Microsoft, which is one of Level 3’s biggest customers has been using VoIP based on SIP for their XBox Live service to enable gamers to talk to one another as well. So now with the integration of Outlook, Messenger and Live Communications Server users of the combined and integrated technology will be able to talk when they want to. Om Malik has a great summary in Business 2.0.