First thing you must understand if is an hardware or software problem. In my case it was an hardware problem. During display substitution proximity sensor was not clean or some glue went above it, or the display was not enough attached to the body (too much pressure breaks the display, too low let it lifted….not easy to mount it properly) I cleaned all the area in the upper part of the phone, i removed the old glue. With the display still mobile i tried an app called “Proximity Sensor Reset/Repair” by Mobile Direction. I did the test (green light means sensor not activated - red light means sensor activated), i did a new calibration, reset phone and it worked. No more blank screen during calls. If i put the phone near or far it blanks or lights up…..problem solved (Moto Z2 Play)