The OCS Client shows an exclamation point. When you drill into the error, you see the following:
Some calls to and from people outside of your corporate network may not connect due to server connectivity problems. Try signing out and signing back in. If this problem continues, contact your system administrator with this information
If you see this error show up in the OCS 2007 Client, the quickest fix is to either stop all OCS-related services on the edge server or reboot that server. For some reason it quits responding to port 5062/TCP from the front end server even though it works locally (ie, “telnet 127.0.0.1:5062”). There are no errors on the firewall so the traffic is passing correctly but the Edge server doesn’t want to establish a connection.
If I see this again I will try to restart the services individually to see which service is the one that hangs.
*Image swiped from Rui Silva’s blog. If my fix above doesn’t help, he has a very in-depth write-up of an alternate fix to this error.