After upgrading the Exchange 2007 server with windows updates, I started getting an error about incompatible security settings when calling exchange UM from my OCS client. The problem lied in the UM dialplan.
Exchange Management Console|Organization Configuration|Unified Messaging|UM Dial Plans tab.
Open the dial plan for the OCS server and check the VoIP security setting. Mine was set to SIP Secured for some reason. Change it to Secured and restart the Microsoft Exchange Unified Messaging service or reboot the mail server.