>> Can’t hack a fax machine.<<
As someone who sent millions of faxes on behalf of others so it looked like it was coming from their phone numbers, I’m not sure what you mean by this. Because what I did was authorized by the sender and therefore legal, it’s not hacking, but the distinction wasn’t in the technology.
I’ll tell you that if I have your signature on a document I can manually put that signature on any document I want and fax it and no one will know the difference. Anyway, scanned documents are legal documents, too, according to my lawyer.
IMO It’s state-actor level work to intercept and pull off a man in the middle attack, to detect and steal a fax going over a switched (POTS) circuit.
Endpoints are the weak link. Including the humans at the other end.
And yeah Caller ID data is meaningless.
The fax contents are pretty secure although authenticating the sender is dubious.
Would you agree?