How do junk fax spammers know a phone number is a fax number?

December 23, 2021 11:05 AM

My church and workplace both have issues with this age-old problem that just won't go away. How do the spammers know it's a fax number even if it's not publicly posted? Wouldn't war-dialing every number for fax numbers cause everyone that picks up a phone to hear a carrier tone (awful shriek), which certainly doesn't seem to be the case? (None of the spam calls I get ever have a carrier tone - they are all either robocalls or hang-up calls.)
posted by Seeking Direction to Technology (3 answers total)

You can buy lists of fax numbers. I imagine they're mostly using those. Yes, someone at some point had to compile that list, but it could have been from a combination one-time war dialing and scraping websites, business databases, phone books, etc.
posted by primethyme at 11:06 AM on December 23, 2021 [4 favorites]

IIRC, neither fax machines or modems shriek out of the blue. The answering side sends carrier first. Fax machines do beep every few seconds so that voice/fax switches can send the call to the fax port, though. I don't think they have to do that, however.

It would be easy enough to dial a call and then send voice if no fax machine is detected.
posted by wierdo at 11:38 AM on December 23, 2021 [1 favorite]

You can read all the gory details in the T.30 standard (pdf). Fig. 5 through 6b on pg.21-23 illustrates the sequences for auto-answering of auto-dialed calls (i.e. no human in the loop). Tones and timing are discussed at 4.1 on pg. 24.
posted by snuffleupagus at 8:41 PM on December 23, 2021 [3 favorites]