When I use my phone my cheek bone makes the phone go on mute or cuts me off, the touch screen stays active whilst I am talking, shouldn't it not work?

Before I updated to IOS7 the touchscreen of my phone would become inactive whilst I was talking on the telephone. Now for some reason if I am talking on the phone my face can set the phone to mute or speaker. It is really annoying as this never used to happen. Surely the touch screen should become inactive whilst you are speaking??