iPhone 5s hangs up during call due to face pressing buttons

My iPhone will not stop hanging up/mashing buttons due to my face hitting them. I just got off the phone with Apple Tech Support, where they told me some bs about pressing the home button and then the lock button to shut off the screen . HOGWASH! I tried again and that hangs up the phone too. Could somebody please tell me how to turn off the screen before I smash my iPhone to bits?

Thanks