Step 1: Using a flat bladed screwdriver loosen the Bakelite telephone case retaining screw (it only needs loosening not removing). See below.
Step 2: The Bakelite case of the telephone should now hinge out from the metal base of the telephone as shown below.
This gives easy access to the metal base.
The Base (or wall plate) has three mounting points.
Step 3: Using a pencil mark the wall through these three holes (one at the top and two at the bottom). Making sure the wall plate is level.
Tip: This is a lot easier if you use help, get one person to hold the weight of the Bakelite telephone while you position and mark the holes.
Step 4: Drill the marked holes and use the appropriate screws and if needed raw-plugs for the type wall you as fastening it to.
Step 5: Once the wall plate is securely fastened to the wall, you can simply close the case (making sure you do not trap the wires) and gently re-tighten the case retaining screw as in step1.
You can now make test calls to your Vintage Telephone and make sure it rings correctly etc.