Repair step
- Go to Settings>Sounds&Haptics. All going well.
- With the screen on, place a call to the phone using a known-good phone. The decline and answer buttons appear on the screen.
- Turn the screen off. Place a call to the phone using a known-good phone. The phone is not responding at all. Press the power button to turn the screen on. We can see the decline and answer buttons on the screen.