Back in the mid-2000s, I used to get calls in the middle of the night, multiple times per night, from fax machines to my fairly new but unlisted number. The ringing would wake me up. I answered a couple of times to try to understand who it is, only to hear the fax tone.

I thought I'd be clever and figure out whose fax machine was calling me, so I could tell them to update their call list. So I installed some fax machine software on my PC and hooked it up to the phone (somehow). Unfortunately, it was all advertising for various scam-like products and services, so I couldn't identify a caller.

Apparently, the number the phone company had given me once belonged to a fax machine. So I called the phone company and changed my number. The calls stopped.