Making the most of a dumb fax switcher box in the old days

Shouldn't feel bad about it, really. It's your/their house, and anyone buying a house (even brand new) should be expected to find all sorts of random and weird junk.

And now telephone lines themselves are rarely if ever even used anymore; the last ones I've seen have just dumped into a DSL modem.

A competent telephone technician with a toner would easily figure out what's going on.

I'm not sure, it took three techs three tries to figure out that the previous owner had hard-installed a DSL filter in the demarcation, and then painted it to look like part of the box.