Answer:
1. Usually fax number is related to the phone number so spammer can easily take a guess. But usually, spammers use a software that generate fax number according to the area code that was inputed. Also from phone directory, the most common way of spamming.d
2. I just want to say that, if you are brothered with the fax spam and it costs you to waste lots of paper, you can consider some fax software, such as Joyfax Server. You can view the fax on your computer and delete the spam message.