OK, upon actually reading my sheet, I see that the device tells you to simply undo one of the 2 phone lines (red wire from the jack) and install the switch, so the phone is deactivated. You can't even pick it up and use it if you hear another phone ring, or initiate a call.
I think the easiest thing to do if you have a 500-type desk phone, is to bend out the ringer loudness stop tab. This allows bells to close up around the clapper and you'll hear almost nothing, but the phone will still work.
Photo 1 - 500-type phone at minimum loudness before stop tab is bent:
Photo 2 - after tab is bent out, allowing one more notch to silence bells:
To permanently disable bells but keep the phone otherwise working, undo one of the bell coil wires, in this case, red.
Photo 3 - bell coil with gray and red wires:
Photo 4 - at L2, small red wire from jack is left attached and large red wire from bell coil is taken off. This is where you'd put the switch to turn bells on/off.
Photo 5 - wide view reference.