I have this symptom on some phones.
When a button is pressed, for example the settings button, the phone does nothing. Also when I try to dial a number. After a few seconds the pressed buttons are executed delayed. After a reset of the phone, the problem is gone for a while.
We are using CUCM 184.108.40.20600-28 and the firmware sip8961.9-3-4-24
Please check the following bug
I have this problem with 8851 with CUCM 10.5.2.11900-3 and sip88xx.11-0-1-11. The issue is intermittent. A phone reset clears it up but only temporarily. Problem was there with sip88xx.10-2-2-16 firmware also. Applied UI adjustment as described. Waiting for results.
Update: UI change has no effect.