Combine Luma Key with a Mask (using the Pen tool). Mask around the physical phone screen first, then apply Luma Key. This ensures that if the actor has a bright white shirt, it doesn't accidentally turn transparent.
For modern workflows, Ultra Key is almost always better. Even if you are removing a white background, set Ultra Key to "Aggressive" and use the eyedropper on the white area. luma key premiere pro download