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Junk fax is the accepted name for a phenomenon known as "fax spam", "ad faxes", and "those goddamn bastards calling my number in the middle of the night and it's not even a goddamn fax number and so i just hear this horrible squeal why won't they just stop calling stop calling STOP CALLING". Junk faxes are illegal, but many companies (including fax.com, the most egregious offender) still persist in harvesting numbers with wardialers and sending out unsolicited commercial faxes.
This illegality is both good and bad news. The good news is that you are allowed to sue the faxer for $500 per incident thanks to the US legal code and a 1991 law called the Telephone Consumer Protection Act (which was, by and large a good thing and is not to be confused with a more modern TCPA, the Trusted Computing Platform Alliance, which is a very bad thing). If you get a particularly egregious case, then the act allows you to ask for triple damages - up to $1500. Under certain circumstances, these suits can be filed in small claims court, which is a net win because small claims courts require you to get a little less lawyered up, and are more friendly to the little guy.
The bad news is that, since junk faxes are illegal, the companies who send them are often extremely hard to track down for you to sue. The mechanics of suing someone, however, are best left for another writeup where someone who is actually a lawyer can give something that is more akin to facts, or one of Nolo Press' excellent books.
If you receive a junk fax, SAVE IT. Put it in a folder somewhere. You might not be pissed off now, but if they keep occurring you might get angry enough, and you can request damages for any incident that has occurred in the previous four years. Not only that, but multiple faxes from the same company shows a pattern of abuse which can push the damages into the $1500 area. Even if you don't want to sue, you can report them to the FCC, and the FCC will take action. Where to send these letters can be found from any of the top Google links for junk faxes.
So why are junk faxes illegal? The answer is simple - you have to pay for toner and paper. Even more obviously than spam (aka. UCE or UBE) they shift the burden of cost from the advertiser to the consumer. I can assure you that I did not want to pay $.05 to receive that ad about some new sketchy health care provider this morning, but functionally I ended up doing so because my fax happily accepted the incoming call. The business they are advertising is closed on the weekends, but I'll find out if they are located in my state on Monday. If they are, then you, dear noder, should expect to hear all about my legal adventures in small claims court...
...and they are not. So you won't.