Most of the time these scam artists are not even in the U.S. They are spoofing a number to make it look like they are here. They do not care about the DNC registry. Consumers Union says we have to get our telephone provider to install blocks on robocalls. The problem with that I think, would be that we have to pay for the technology and the blocking.
They do not even know if you have a credit card. They are assuming since most people do. So it doesn't matter if you have a balance or not. They do not know. All they want are some credit card numbers so they can make bogus ones and sell them. I have also "played" with them by taking forever to find my number. After that I say do you know about the DNC registry and they hang up. Then they harrass me by calling every 10 min with no name or phone number on my caller ID. Therefore I cannot block it. I found out it is better to not even talk to them, just hang up when you get the recording at the beginning.