These people are like viruses. Turn it around on them - send faxes to the number that called you to tie up their lines and drive them crazy. you won't get charged for it, 'cause it won't go through, but they'll get that annoying squeal if they answer.
I filed a complaint with the DoNotCall folks about a different telemarketer. I included a link to this site and asked them to please prove they are not just a bunch of people collecting a pay check and to nail the telemarketer. One of their people actually posted on this site and explained he was familiar with that telemarketer and explained what research he had already done to stop them, unfortunately to no avail . . . yet. However, that telemarketer was still in his crosshairs. Definitely file a complain and include a link to this site.