Poco M3 – Screen off while incoming and out going calls

You need to give more info. of the problem that you're facing. The phone screen should be automatically turned off when you pick up a call and lift it near your face.

If it doesn't do that, you can enter *#*#6484#*#* on the dial pad to open the Hardware Test menu, choose "Proximity sensor" and follow the onscreen instructions to test the proximity sensor.

If your test fails, it could be an issue with the sensor itself, then you can visit the nearest Xiaomi Service center to seek help.

If your test passes, but the proximity sensor doesn't work as it supposed to (screen turns off when the phone is near your face), then you should report this issue in the official Xiaomi forum, https://c.mi.com/global/forum.php

Tip: If you're unable to disconnect calls with the screen is off, you can press the Power button to do so. Make sure this option is enabled in Settings > Additional settings > Accessibility > Physical.