Another oddity. The dial tone was 90Hz, a frequency that had extremely low transmission properties across the physical wires to the phones. The lines we either weighted in frequency from 300-3000Hz (3kHz Flat) or psophometrically weighted around 1 kHz. Regardless, the 90 Hz ring tone didn’t pass through the wires. How did it get through? They used two tones, 250 and 340 Hz - when mixed together produce intermodulation products, one of which is 90 Hz. It is the human ear that is the
‘mixer’ and the brain perceives that one product the most because it’s the lowest. Neat, huh?
The hole cult of ‘phone phreaks’ fascinated me when I read about it - including the fact that it seemed especially attractive to some blind people:
https://www.eetimes.com/the-blind-hacker-who-inspired-apple/