Business is too fast paced for you to have to rely on phone lines to fax. They are expensive (at $20-30 per line average) and shouldn't still be one of your concerns in the 21st, cell phone filled, century.
Fax is a safe transmission method. There can be no "man in the middle", the information is encrypted by nature and it is direct one-to-one communication. There's a reason the health care industry likes it so much. The only problem, it hasn't been updated in 20 years.... or has it?
Faxing can now be analog or digital and on premise or cloud based. POTs lines are no longer required and a phone network is no longer needed. Give more time and accessibility to your employees by letting them send and receive documents from their email client or web browser. Keep users mobile by securely faxing from a mobile device.
Faxing has come a long way in its lifetime. Starting around 1843, developing into FoIP in 1998 and still evolving to this day.