While screen protectors can cause issues with the sensor usually they cause it to turn off and not come back on. They get dirty, soiled, gap at the edges or something gets under them then they think they are seeing dark all of the time causing the screen to go off immediately or not come back on once pulled away. The issue is more likely that your hair is stopping the sensor from seeing the dark or intermittently allowing light in allowing the screen to come on. Then you accidentally bump something that puts the phone in airplane mode. I'm not sure what in the phone app could cause this so it's my guess that you have a widget on your main screen to turn on airplane mode? If I'm right about either of these the two things you should try is brush your hair back from your ear before talking, if your hair style will allow this, and/or move or remove the widget from your home screen. The latter may mean that something else will open or happen in lieu of airplane mode being turned on, but it may not end your call. Also if you wanted to test this to see what exactly is happening, try making a call and simply cover the sensor with your thumb. If the phone goes dark you know it's working the way it should, and if it doesn't you will know your sensor is not working correctly. In this way also you can see if it is simply being on a call that is turning on your airplane mode. Either way if the screen doesn't go dark or it activates airplane mode you'll be able to take it into a center or activate your insurance.