How does Microsoft Exchange Server 2010 Unified Messaging respond to certain outage scenarios? Here’s a quick look, but be sure to click the link at the bottom for the full details…
Mailbox Server Unavailable
If a Mailbox server for a user is unavailable, the UM server will continue to accept calls on behalf of the user. But the user's custom greeting won't be played. Instead, a standard greeting will be used for calls to that user
Hub Transport Server Unavailable
If the Hub Transport server is unavailable, the UM server will continue to accept calls and queue the calls, depending on how you've set up the queuing limit, until the Hub Transport server is available.
Domain Controllers Unavailable
In a situation where all domain controllers are unavailable, the UM server will be unable to accept calls.