I had a problem last week where my phone started to lock the screen the moment I clicked on dial. It seemed that the proximity sensor on the OnePlus 3T was stuck and it assumed that I had the phone up to my face. I contacted OnePlus about this issue and their solution (As Always) was to reinstall the entire operating system. Basically wipe the phone.
Well, the simple fix is to toggle double tap!
So, to do this, go into your settings and find Gestures. In Gestures, turn off Double Tap. At this point, I rebooted my phone, made a phone call and verified that the problem was gone.
Then I turned on Double Tap again and made another call.