I have another idea, based on a cliff-jumping technique (which I can't remember the name of).
1) Decide on a suitably tall building that has roof access.
2) Get a rope that is a meter shorter than the building is tall.
3) Build a sturdy case for the iPhone, attached to one end of the rope. The case only needs to have one small hole for the camera lens. (I would build it of paper or cardboard, and then wrap it with duct tape.)
4) At the top of the roof, one person holds the camera and that end of the rope, at the edge of the roof. Another person holds the camera and moves SEVERAL METERS AWAY from the person holding the rope, also standing at the edge of the roof.
5) When the phone is dropped it will only free-fall partway down the building before the rope becomes straight. HOWEVER, since the person holding the rope isn't directly above the falling iPhone, the rope will more GENTLY swing the phone to the side instead of stopping it abruptly.
Test this first with something that weighs the same as your iPhone but isn't as valueable (like a piece of wood or a small book). Figure out where to stand so it doesn't hit the wall when it's swinging.
I hope this works! Good luck!!