Step 1: Measurements
For the telephone, the author measured the inside of the frame around the off-hook button, the depth and the location of the switch. For a different kind of project you may need to measure the position of the button switch you want to control, than transfer the co-ordinates to the top plate. You would also need the depth from the top to the button when pressed.
Step 2: Basic Principle
Using a magnet instead of springs has real advantages. It reduces the wear factor and maintenance requirements. It also simplifies the design.
The idea is to align the screws (shown as white dots in the picture above) so that they touch the magnet when toggled forward. They don't have to touch at exactly the same spot but must touch on the magnets strong surface. If the magnet has a hole in the middle be careful not to line up over the hole. The magnet is weaker even with a screw in the hole (metal screw not ideal) .
The two screws in the toggle part should be made of a metal strong enough to feel the pull and snap and hold when tested. The author used a N52 Neodymium bar magnet I had from another project. I aligned both screws to barely touch one side away from the screw in the center of the magnet.
Below is a video of the model alignment. Just make sure each screw face is vertical when the toggle part is in the locked position.
Step 3: Toggle Bar
Use a metal rod to go through the toggle part for strength. The author use a 3 mm brass the length of the box. A nail could be used to secure the structure. Just make the holes large enough to accommodate it. The author threaded the ends in the prototype but found it wasn't necessary in the finale unit. If your holes are lose just heat up the PLA and it will expand to make a tight fit. The rod does not turn so there is not much stress on it. You can add some glue or cover the ends if needed for your application.
Step 4: It Works
The toggle switch created was large enough that one could just swipe it with any part of a hand to flip it in the other position.
Step 5: Assistive Pointer
In case you are interested, the Tee used is a common device for assisting one who has a closed fist or similar constraint. I made one by taking a 4" section of 1" PVC and filling it with scrap PLA; just heat and push into the PVC. Make sure you fill the middle section solid, so the worm screws can set. The author put two holes one on each side and two worm screws, than heated the worm screws with a heat gun and screwed then in. It made a tight fit. You could probably just use one screw or none if you didn't plan to change the pencil out. A regular flat head screw would also probably work; just counter sync to fit flat. Different length pencils are needed for different applications. If you need a bigger tip you can add a pencil top eraser over the pencil.