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. Cha
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nge it to Secured and restart the Microsoft Exchange Unified Messaging service or reboot the mail server.