We have experienced this on occasion-- someone would leave and their user either disabled or not would remain in a queue. Their previous phone (which had been reassigned) then acts disabled, even after reassignment and resync.
The exact conditions this last time: parts user left, their phone was reassigned. The user was moved to the new parts queue and assigned the phone. The phone was renamed and then resynced.
When the call queue was called, the phone would not ring. Only after the old user was removed from the queue would the queue ring the old user's phone.
The old user had a disabled softphone associated with their account. We had been told in the past that disabled accounts in queues were a problem.