Why is screen sometimes unresponsive when receiving calls
This problem is quite known for sometime for touchscreen devices. Most of the times this happens when the screen is under some pressure during touchscreen recalibration after being woken up by the incoming call (maybe being inside pocket, etc.).

The only solution for you is to disable the option, power button instantly ends call by going into Settings -&gt; Accessibility -&gt; System

If your screen becomes unresponsive to a call, just press the power button (to lock your screen), press it again and unlock the screen (with your pattern or PIN, etc.) Then you will be able to answer the call.

This solution is applicable to any situation where the screen is unresponsive.