The do-not-call list is a joke. These companies simply ignore it. How do you sue them? I have done it successfully in the past (google "Transcontinental Warranty FTC"). TCW was shut down and the owner paid millions in fines (but I got nothing, other than the satisfaction of doing something.)
You need to get on the line with them (and record it) and get as much info from them as you can. They do not want to give out their real name, so you have to gain the trust of the caller. Once you have as much info as you can get (including the real company name), go to the FTC website and fill out a complaint. If you're lucky, and the FTC is already investigating them, you'll get a call from the FTC and have the joy of submitting to endless interviews and depositions.
They are looking for LIVE numbers to resell to MANY other companies. If you answer its LIVE. If a voicemail they dont record it, they just keep RECALLING the number HOPING for a live response to resell the number on a demographic map.