I was looking for the name of the button on a telephone that you push to hang up. On older phones where the receiver sits horizontally over two buttons, I've seen them called "plungers." Are people familiar with this term? Is there another term? Is the single button also called a "plunger?"
The American Heritage Dictionary of the English Language, Fourth Edition, gives one definition of the word plunger as:
A machine part, such as a piston, that operates with a thrusting or plunging movement.
So it's possible you might be able to use plunger as it somewhat fits that definition.
Frequently used technical terms for that part are switch hook and hook switch.
It is called a Switch Hook. This is where we get the terms "on hook" or "off hook" for phone states. Example; "My phone was ringing off the hook!"