lock screen issues

When I am on a phone call with my Q10, the screen will change to the phone key pad and in most cases I accidentally move my phone while talking pressing on the key pad, hence the noise of pressing a key on the key pad.

The screen should stay on the contact (the call I'm on) screen and not change to the dial pad screen. I can I correct this? What feature do I have turned on that causes this.

Normally, your proximity sensor should kick in when you're on a call an and the phone is held close to the ear or any surface for that matter. So if it doesn't, then it might be bad (the proximity sensor)