So, again, I finally played along. Got a big huge quote for all sorts of windows and doors our house didn't even have. Told them that I'd just send them a fax photo of one "really weird" window so they could look at it and quote me properly on that. Got THEIR fax number. Promptly hung up, logged into VoIP management portal...and redirected all their own faxes right back to their own incoming fax machine.
For years and years....every time they tried to fax-spam me, someone...somewhere, was spamming themselves. I saw it in the call logs.