The client in question is using CUCM 7.0 and has a 7941 IP phone. We've been seeing some strange intermittant behaviour.
When the call is connected it will sometimes be immediately placed on hold and then drop off the phone. The caller will be left on hold but the call has dropped off the phone so the user can't resume the call.
If the caller then hangs up and calls back the call will (usually) connect normally.
I have not been on-site when this occurs but I can be reasonably certain that its not an issue of user error.