Welcome to Android Central! When exactly are you seeing this? Is this when you're in an active call, and then bring the phone down from your ear to enter a keypress? If so, this kind of thing occurs with a phone that doesn't have a proximity sensor. Phones with proximity sensors will automatically turn off the phone screen when you're in a call and the phone is against your ear (to prevent your face from accidentally pressing buttons on the screen). If the phone doesn't have a proximity sensor, instead it will lock the screen as soon as you start the call (which will also prevent your face from accidentally pressing buttons). In order to unlock the screen during the call, you have to double-tap it. I don't think there's any way around that.