Yesterday when I tried to make a call to call one of mine Exchange Auto Attendants the call mysteriously failed. At the same time call to other Auto Attendants went just fine. The more confusing part was the error I traced with the Lync Logger:
The Unified Messaging server cannot find a valid UM hunt group associated with the UM IP gateway. Pilot number: <AA Name>.<UM Dial Plan Name>. UMIPGateway: <The Lync Pool FQDN>.