Your method of giving them the number from an old gift card or empty debit card is good for wasting their time but reporting them to the FTC does nothing because despite the federal law against such calls, the FTC simply will not bother investigating any of the millions of reports that they get -- As least not unless the prefix in front of your name happens to be Rep or Sen.
If the FTC actually gave a damn about enforcing the law "Card Services" would have been out of business at least a year ago.
I just got a call withteh same thing. I said "no, take me of the list, this is a business phone." Then he repeated what I said. I said "I'm not interested" and he repeated that. I said hello and he repeated again. lol!!! I hung up after that.