When I'm on a call and need my hands free I use my shoulder to hold the phone to my ear. Frequently the call ends from whatever part of my face, chin or cheek hits the end button. I've tried the "Power button ends call" but when I test that function it doesn't work. Does anyone know how I can lock the screen during a call or how to properly make "Power button ends call" work? Thanks a bunch! I have a Droid Ultra...