I just experienced a problem with our CUCM 6.1.2 All my phones went dead suddenly. Internal calls could be made without issue, however outgoing calls received long dead space followed by a busy signal.
The real kicker was incoming calls. Incoming calls would not real ring until minutes after. Of course, by the time the call rang our the phone the caller had long hung up and the recipient was greeted with silence.
Good news is that rebooting the system seemed to right everything. Bad news is, I have no idea what happed and if this issue will arise again.
One other thing to note, I upgraded from 6.0.1 a week ago. No problems after the upgrade till now.