Phone screen won't display while on call if phone moved away from ear

The usual reason for this is a screen protector which is covering (and thus, confusing) the phone's proximity detector...

This is the case (usually) when I see this issue. The screen protector is either on wrong or cut badly. I would find where the sensor is on the phone and see if the protector is covering it -- if so that is most likely the cause. I would just take it off and make a call to a friend or something to test.
 
I will admit, it does seem counter-intuitive, since glass is clear. It wasn't until I pulled my protector off my Nexus 5X, that the sensor worked like it should.
 
I have the DT2 that is just like I took it out of the box. I have not added any screen protectors or cases to it and I have experienced the same issue as THEPRFCT10. On some occasions during a call when the phone is removed from my ear the screen does remain black and you cannot get it to come up by pressing the power button or anything. There doesn't seem to be any rhyme or reason to why it happens and it doesn't happen all of the time, in fact for me it does work like it is supposed to most of the time. Just adding my experiences with a DT2 that has had nothing added to it.
 
I will admit, it does seem counter-intuitive, since glass is clear. It wasn't until I pulled my protector off my Nexus 5X, that the sensor worked like it should.

The proximity detector uses infrared light which can be either absorbed (no pass-thru) or reflected by different types of glass. Thus, it renders the detector useless (the light is absorbed and never bounced back to the receiver) or the detector is in a constant triggered state caused by the reflection.
 
I had this problem you need to put phone in safe mode then turn phone off. I don't remember exactly how I did it but Verizon tech suggested it and it worked this is done first before doing the hard reset.
 
I had this problem you need to put phone in safe mode then turn phone off. I don't remember exactly how I did it but Verizon tech suggested it and it worked this is done first before doing the hard reset.
Hmmmm, no idea how you put into safe mode before powering off?
 
I have a LG K7 and am having the same issue. My screen protector does NOT block the sensor, it actually only covers the part of the screen that lights up. I have had the phone for about 3 months, and this issue only started about 4 days ago. I have tried to recalibrate it's motion dectection and am having no luck. It also makes a bunch of touch tone sounds with a black screen and nothing pulled up to make those sounds. I have warranty and insurance on this phone, but was hoping someone would know how I may troubleshoot this issue. I tried to shake it while I'm on the call, but the screen doesn't come back up unless I hit the power button. It also constantly disconnects my calls. Is this a issue I can solve on my phone, or do I need a new device?