Screeeech . . . hisss . . . squaawk. These are familiar sounds to anyone who's ever used dial-up internet or called BBSes. It seemed especially noisy late at night. Have you ever wondered why all that noise was necessary? And did you know you could have muted your noisy modem?
Of course we knew how to use ATMx, where x could be used to mute the modem (0), let it speak while establishing the connection (1), and all the time on (2).
I was usually using it with x=1 during the day, and I had a script to mute it during the night, especially since I received automated calls during the night in the eighties and early nighties (UUCP on AmigaOS, then on UNIX).
Fixed IP networking came to me in 1996 (cable modem), and I abandonned UUCP and FTN (ifmail) over TCP in 2003. I think I disconnected my last fax modem and UNIX login in 2012 or so, as I abandonned analog and ISDN telephony for 100% voice-over-IP.
You can find audio and explanation about what the modem noises were here: