The Do Not Call list is only helpful for eliminating calls from legitimate marketers, who want to comply with the law. Scammers and fly-by-night marketers use random dialers, and would never bother checking the list.
I also use your technique of adding known-bad numbers to a "Scam" contact on my phone. It's highly unlikely that you would receive a valid call from that same number in the future, even if someone legitimately purchases it later, and in the meantime it makes it easier to ignore.
I received a two-ring call to a private mobile phone number with a Washington State area code. I attempted to answer but heard nothing on the line, so I hung up. Based on the information others have reported here, I will definitely not be calling back.