I don't know what kind of magical coating they used on top of the black-painted iron (or maybe steel?) box that the thing is made of, but I would like it to go away so I can decorate the thing properly. Maybe it's just car wax? Maybe it's something in the paint itself?
I'm hoping for an answer that is less involved than "disassemble the entire thing, sand it and repaint it". Taking it off the wall is a giant pain in the ass.
It's ironic, because the payphone is literally the only thing in the building I want stickers on, and they stick fine to absolutely every other surface.