“This just one of the things about the old analog phone system that were very clever. “
You want “clever”, look into the original color TV design. Done initially with vacuum tubes (not thousands of transistors), and the color signal would work with the old black and white TVs. And the sets were manufactured at consumer prices.
And the picture tube was very complex with three guns that shot three beams through a mask to illuminate thousands of red, green and blue phosphors.
True. No argument there.
TVs were one of the first places where phase-locked loops (PLLs) were used in a consumer product.
Color TV depended on PLL technology even more.
FM Stereo wouldn't have been possible without it.
The PLL was one of the great innovations of electronics, incredibly useful and versatile. A case where the whole was much greater than the sum of its parts, the added ingredient being cleverness.