That's a pretty vague description, but I'll suggest looking at a few things. If it's something that just stopped working, check the OCS Application logs on each server as well as the Application event Log on the each workstation attempting to send/receive voice calls. There should be a number of errors among all the endpoints to point you in a general direction.
Then validate that some/all of the server accounts passwords have not expired. If so, take a look at this article for assistence in resolving:
http://blogs.pointbridge.com/Blogs/schertz_jeff/Pages/Post.aspx?_ID=66Also, verify that none of the certificates in use by OCS had expired.
Jeff Schertz, PointBridge | MVP | MCITP: Enterprise Messaging | MCTS: OCS