I had this EXACT problem.
Back in the days of modems, my parent's house kept getting lots of fax phone calls. We didn't have a fax. Most annoyingly, when it started, it would go on all day long, every five minutes. There was no way to identify who it was or stop it.
So, while I cobbled together some fax software for the modem, my brother phoned BT. They intercepted our phone line for an hour and then called us back. Turns out, some automated bank system somewhere had the wrong fax number plugged into it and it was sending us all the inter-branch faxes with customer details and all sorts. Damn shame I couldn't get the fax-modem working in time to collect it myself.
The BT engineer obviously had access to their phone lists and called their head office (because ringing back on the number that was sending was something we'd already tried and we just got a fax answer) direct. The problem was sorted in minutes after that because of the nature of the material, and the harassment of ourselves.
Nowadays, it would be a huge data protection fine. And it would have been only a few more minutes before I'd built a fax-modem that could receive them and then sent them to the head-office myself. The practice is still going strong, by the sounds of it, and they are still just as inept when it comes to typing in the phone numbers of the destination.