Sadly, criminals do not care about the law. Its like complaining about people who drive slowly in the left hand lane when the law forbids it. They don't care. The only people who are abiding by the DNC list are most likely legitimate businesses who operate within the law out of their own ethics or fear of government legal consequences. Googling the number and then blocking them once you determine that they are illegitimate is pretty much your only option worth considering IMHO.