I have a Cisco 8851 phone, that is registered to CUCM 10.5 and worked fine for a year. Phone restarted and getting " Device is out of Service" error. Any assistance greatly appreciated.
Try to upgrade the FW on the device, you can also try a factory reset, if no avail, I'd call TAC
In addition of what Java suggests you may also check, by using the IP address of the phone on a web page, the Device Logs to check if there is anything useful.