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.

POTS didn't have an opus audio codec.