Has this phone been used before on a CUCM cluster? You may want to try resetting the security settings on the phone: Settings -> Admin Settings -> Reset Settings -> Security Settings -> Reset
Two more thoughts:
- Have you looked at the logs on the phone? They might reveal something.
- Power off the phone, delete the device from CUCM, re-add the device and try plugging the phone in again.
you can refer the link.
what is exact CUCM version and firmware under Device ---device settings ---device defaults for 9971?