Fixed for me to on a Nexus 5X: Phone screen would go blank as soon as i dialled. Plugged headphones in and screen lit up. Found this page, then swapped BOTH proximity sensor bits from old screen to new. Was left with intermittent "blank screen during call" fault. Then removed "new screen" plastic shipping cover that I'd left on as a temp screen guard. Apparently that was enough to trip the proximity sensor. all good now