Anyone talking about "fax security" is another monk of security through obscurity.
Phone line can be listened, fax can be hacked[1] and, most of the time, the fax is the copier and everyone in the building has access to it.
20 years ago I worked for a client that had a fax to exchange connector, any mailbox could send a fax from outlook[2] and they had linked fax numbers to group mailbox so that each department had their own fax numbers.
1 : https://blog.checkpoint.com/security/faxploit-hp-printer-fax...
2 : It was always fun to analyze fax sending errors because someone wanted to send a funny video to a fax number.