OMG! I’m so old. I even once figured out how a dial tone at 90 Hz ever made it through the hard wires to your ear in that handset. Turns out the lines (~300-3KHz) couldn’t transmit the 90 90 Hz we normally hear but the human ear has a mixer effect that tends to emphasize the lowest intermodulation product of two tones. The actual tones were a combination of TWO tones: 350Hz and 260 Hz ( during my time) and later 350 and 440 Hz later. The difference (an intermod product) being 90 Hz.
Just a useless fact now.
Not to me. I measure intermodulation, albeit at very very much higher frequencies! When you 'mix' two frequencies, audio, HF, RF or microwave, or even light, you get the two originals plus their 'sum' and their 'difference'....................