When you pick up a landline phone, throughout the USA there is a consistent "dial tone" you hear in the earpiece.

I haven't heard it in a while, but IIRC it's two synthesized tones roughly a "major third" apart (https://en.wikipedia.org/wiki/Major_third), slightly buzzy-sounding, which means they probably weren't a "just" major third (https://en.wikipedia.org/wiki/Just_intonation).

Once you started dialing, the dial tone would go silent. I think the dial tone would remain in place if you spoke, but I don't remember for certain.

Does anyone know how this worked? What generated the dial tone? Presumably there must have been a network of machines responsible for processing people's dialing and routing calls appropriately; did these machines also generate the dial tone? If so, were they all made by the same company, Bell-something-something? The dial tone was/is so consistent in its sound and behavior that it seems likely.

Just curious if anyone knows or has links!