Hi, am also having the same problem. Tried the suggested Reset and Proximity Check. Now running on Safe Mode. Till now no difference. Shall update.
Update: The problem is not there while running in Safe Mode. But as soon as I switched back to Regular mode (after 30min or so) the problem was back. So seems like a conflict with some app. I am having exDialer app , but the issue was there with the stock dialer also. Any suggestions ?