I don't know if this is the professional way to unbend a phone, but I used a wooden cutting board and rolling pin! the wood was hard enough to bend the chassis, but soft enough to disperse pressure evenly!
I know you posted this awhile ago, but if anyone else is wondering, how to fix the issue where the display won't turn off while on a phone call, You MUST transfer over the small plastic retaining brackets from the old display, and put them in the correct position! By correct position, I mean the two small transparent points need to line up with the receptors on your iPhone's front panel assembly (Camera, light sensor, mic).
