Would recommend gathering TVS logs and opening a TAC case. Its usually IP phones trying to register but failing and ultimately leading to TVS threads being exhausted. You'll see something like "debug ERROR:tvs_wheel_handler: thread may be stuck in ha...

Though this post is old, answering it so anyone else seeing this can refer. If a factory reset doesn't do the job, try to push the latest firmware for the phone and see if that resolves the problem. If the issue persists on the latest firmware too, i...