If you answer do you get a recorded message? If so listen to the entire message as there may be an option to press "2" or whatever to get removed from their call list. I know it's a pita, but it may be your only choice.
I just called the number and the first thing you get is the option to press "one" to be put on their do not call list. They're calling your cell number already, so what do you have to lose if you call from your cell and press one??? Other than one unit of airtime.
usac.org is a great place to blow a whistle!