When I call someone my display become dark

[AC Question]

My phone is Motorola Nexus 6, sometimes when I call someone from my call list my display gone' But ringing going on I can talk with the person when he received. But I can't do anything till the call end and I switched on through power key.

 

[ManiacJoe]

Near the top of the phone's screen should be a proximity detector. It is used to darken and disable the screen when the phone is put up to your ear during a call.

It is possible that the detector is blocked or dirty or malfunctioning.