Iphone locks while I am in a phone call

For a few months, my iPhone locks while I am in a phone call. That means I can't do anything with the phone (hang up, go to another screen, go to speaker phone, etc because my phone is completely locked. The only way I can get back to my phone is by pressing power button. I have most recent software (I have automatic software updates).

Very annoying. Face recognition won't open the phone. It is just completely black screen and locked, but phone call continues. My only option is to press power button. Any ideas on what's happening or how to fix?