Welcome to Android Central! The proximity sensor is right next to the front-facing camera, so if something is blocking it (like a screen protector or case, as mustang7757 mentioned), it will make the phone think it's up against your ear during a call, even if you take the phone away from your ear. The proximity sensor turns off the screen while in a call when it thinks the phone is against your ear, so that you don't accidentally press any onscreen buttons.