it has to do with the proximity sensor, usually when you have flip to mute enabled in the my moto menu/app it shuts off the display in order to save power and mutes the incoming call.
this can occur if the adhesive around the proximity sensor loosen and form a gap between it and the glass or ge debris or residue get on the inside or outside of the glass.
the solution i have found, although a bit barbaric is to apply pressure AROUND the proximity sensor and whilst doing so taking the call.
if this doesn’t help wiring a headset and taking the call with the remote is an option