Hi Sid,
The top layer is toughened glass (the IOR is pretty irrelevant on a flat piece of glass, so close to the display, maybe just keep it at or around 1.5) and that is covered in grease from fingers, with dust stuck into it.

Practically, you’d just look at a phone and try to reproduce what you see.
Realism doesn’t come from getting the numbers (like IOR) right.