More than 100 billion fax pages are sent each year. Traditional fax comes with a number of security risks. Unlike email, you cannot recall a fax document once it has been transmitted successfully and printed on the recipients fax machine. Fax servers are usually not encrypted either, so as long as someone can access the drives, they will be able to see copies of past faxes. The risks of using traditional fax technology are not insurmountable. Changing fax procedures and introducing physical controls can go a long way in making sure faxes do not land in the wrong hands.”]
Source: https://gbhackers.com/7-security-risks-of-using-traditional-fax/