fyi - When you call these jerks back, you have just made it okay for them to hound you forever. All they need is that ONE call from you (even if they say it's to get off their call list). Then they can claim you have a previous business relationship, because they now have a record of YOU calling THEM. Duh.
Yesterday, I called the home number of a local friend who just moved out of town and recorded that telephone company recording that after the short musical tone says, "We're sorry, but the number you called has been disconnected or is no longer in service. Please check the number and dial again. This is a recording". I intend to play this message to all future incoming calls from "UNKNOWN NAME" callers.