OCS shines with Office Communicator 2007 front end and Exchange 2007
A great review of OCS is available Network World...
Microsoft is getting ready to make a big splash into the VoIP pool with its OCS 2007, expected to ship this summer.
When OCS's software-based VoIP engine is coupled with Microsoft Office Communicator 2007 client software, users have access to IP telephony, instant messaging and presence awareness. Furthermore, when Microsoft Exchange 2007's unified messaging services are added to the mix, voice mail and autoattendant features also are on the table.
More at https://www.networkworld.com/reviews/2007/062507-ocs-test.html(registration required).