Last week, we got a call from a service similar to that described above, with a "spoofed" caller ID from Zeeland, MI near Grand Rapids. The number shown was 616-613-2227. Since we had family in that area, we (made an erroneous judgment and) answered the call. The female started on the "lowering your credit card interest" routine. Advised her that we did not pay interest, as we paid our bills promptly. She continued on, getting progressively nastier. When I advised her that we were on the National Do Not Call list, she went ballistic and stated that they "paid no attention to that". I told her we did not want to be called again and that we would report her. She shouted profanities and said we'd "regret that".
We then blocked that number, but have since been bombarded by calls from the following numbers: 703-493-1172, 703-646-0725, 703-493-1453, 714-607-6298, 866-463-3867, 866-431-5117, and (today) 616-980-2030.
It is apparent that these folks wreak havoc with people who object to their low-life tactics.