ending call options

hey,
so the phone does the great thing of recognizing when you're using the phone on a call but if you hold the phone with you shoulder or just not right against your face it activates the screen and you hang up on people. does anyone know how (or if) you can turn that function off so that if you want to activate the screen during a call you have to press a button or physically activate the screen? thanks

 

i have the eris. i want to make it so that the screen doesn't activate when you pull the phone away from your ear.