Hello,
You can use the Event Monitoring tool, http://www.ciscounitytools.com/App_EMS.htm
To notify you of a failover. Setup a trigger for event
1033, Information event, on the primary, source is CiscoUnity_DSAD and do the same thing on the failover, but for information event 1050, source is CiscoUnity_NodeMgr.
These two events only occur when a filover occurs.
Mike