> M0 or M Speaker Mute, completely silent during dialing M3 is also common, but different on many brands

As a teenager with my first modem (I think I started with a 14.4, so not that early) I had no idea how to mute the modem with those commands yet while connecting to BBSes.

So I created my own "mute" (for during connection negociation at the beginning of a call, the modems were extremely noisy) system...

Late at night, when my parents were asleep, I'd unscrew the cover of the modem, take the PCB out and muffle it between two pillows while the connection was establishing.

Worked flawlessly.

As I was both up to no good and because I didn't want risking shorting the PCB, I'd then put the cover back on. Rinse and repeat every night.