The phone on my desk is a 1980s era ASCOM Berkshire Select. I wanted to use it on a conference call today so I wanted to be able to mute the phone. Unfortunately the only three buttons (beyond the number pad) are labelled R, LR and S, none of which sound promising for mute functionality. A web search revealed that the "S" button is, in fact, the mute button. Why "S" you ask? Because it stands for "Secret".
While I found that tidbit amusing, I was not amused by the actual functionality of the Secret button. Modern phones toggle on and off so you can set it on mute and ignore it until you want to contribute to the call. The Berkshire series, however, requries you to hold down the Secret button whilst discussing your secrets. I was about halfway through designing a complicated system consisting of some tape, a pound coin and my RAPESCO brand hole punch to keep it depressed, but allow me to toggle it off, but instead I decided to use the phone in Lon's office (he's out of town).