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This question is over my head, but it is a good chance to discuss what little is known about telephone technology.
Basically, all audio transmission begins and ends with analog, since speech is inherently analog, and human hearing is analog. Therefore the “front-end” of all telephones start with analog, and at the “end-end” all audio transmissions come out in analog.
The in-between transmission is a whole other question.
An ordinary POTS (plain-old-telephone-subscriber) line is analog tuned for about 300 to 3,000Hz, but a second phone line can be piggy-backed by applying a high frequency (analog) radio signal at about 10kHz which is AM modulated (analog) with an audio signal, and later demodulated back to base-band audio.
Even though that’s all-analog, it still requires specialized handling to hear the “sub-carrier” signal.
Convert to digital and I’m gone. I do know that there are probably countless digital protocols, maybe some are proprietary. Is there such a thing as a specialist who understands the subject in a general way?