The hack is getting the unsecure system not to damage your encrypted signal, to carry even though it is expecting plain voice talking rather than a stream of binary digits.

We’ve been doing that for dialup internet for decades.

Dialup actively co-operates with the telephone system - e.g. the screeching at the start is designed to disable echo cancellers and other such mechanisms.

Dialup doesn't work over every phone line, especially over sat voice lines.