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?
To STEAL a legit fax? Yeah, I’m not talking about that. But you can append caller-ID, legally if you’ve received permission from the person you claim is sending the fax.