Nice rabbit hole.

I was installing Linux and I wanted to have mono audio. So I installed PulseEffects. For curiosity, I clicked on a wave icon and it turned out to be a functionality to test signals. I set the frequency to 440hz and turns out it's the frequency you hear when you pick up an "old" line phone.

https://zippyfacts.com/why-is-the-dial-tone-frequency-an-a-or-440-hertz-on-telephones-in-the-united-states/