Here's a new one to us:
Customer reported a certain 9508 is missing the "Hold" key when a 2nd call starts ringing through. Apparently if the user presses "Answer" the first call is immediately disconnected. All the typical settings existing under Telephony - Reserve Last CA, Call Waiting, Answer Call Waiting on Hold, etc.
We tried replacing the set and changing the port but neither made a difference. It was finally resolved by deleting the user and re-creating it again.