So I ran into a small issue with an OCS environment recently. All of the MOC clients were showing a small red exclamation point in the tray icon with a message indicating server issues and limited calling. Combined with that, 4 of the OCS services were stopped and wouldn't start. The 4 services were: Office Communications Server Conferencing Announcement, Office Communications Server Conference Attendant, Office Communications Server Outside Voice Control, and Office Communications Server Response Group. Some Google searching revealed a lot of people running into similar issues (but with different symptoms or slightly different error messages). I was finally able to narrow down the fix when I ran into Event ID 33020 stating that the CAA's private contact object is missing.
The fix? Deactivate all 4 applications in OCS and reactivate (using the same information that they were originally activated with). This is a very quick and easy process. Then, start the 4 services. Thereafter, users had to log out out OCS and log back in and the red exclamation point was gone and full functionality resumed.