I get asked this quite a bit from schools as they want the ability to place/receive voice calls on the Mac or Linux using OCS R2.
The answer is yes if you leverage the SmartSip technology from our partner Evangelyze.
You can make and receive voice calls natively on the Mac using a SIP softphone of your choice such as X-lite. This would also apply to any other platform such as Linux. The other value here is you can leverage your existing investment in any existing SIP phones on your campus and connect to OCS voice as well.
Here is a cool demo of Mac Messenger 7’s native capabilities with OCS (IM, peer to peer voice calls, video) along with a Mac OS native voice demo (using a free SIP softphone) connecting through SmartSip where the Mac user has been provisioned for OCS enterprise voice: