One small suggestion I have, which may already be obvious to some, is the mounting slots on the back are keyed and if you have a stud in the wall you can line up the right-hand top slot and the screw for the bottom slot so they go into the wall stud. The screw ends up in the narrow part of the keyway and you can easily put a pencil mark where the screw goes. You can also use a sheet of paper to mark the screw locations and hold that to the wall to drive the screws. Then with the screws in the wall, and tearing off the paper, you place the top slot on the screw first and then the bottom one and you can level the phone with that horizontal slot and tighten down both screws. You can add the second screw in the top if you like, but the stud may not be wide enough to accept both.
I'll never forget my grandmother's black 554 that was mounted by Southern Bell. It was on a wall in her kitchen that was shared by the basement stairs on the other side. So every time I went to the basement I could see the station wire behind the phone, stapled to the wall with those tiny staples.