Take a look at this from Mike Sandmman:
https://www.sandman.com/lineshar.html Distinctive ring uses two numbers on one pair of copper. One copper pair into the box and two outlets, phone and fax for instance.
If you dial the 1st number, it rings to the phone, if you dial the 2nd number it rings to the fax.
No matter what you only get one call, voice or fax, at a time.
Verizon is about $6 a month for the second number and if you get few faxes a month, its much cheaper than a second line.
