I'd say you're guessing wrong. The DNC registry does exactly what it's designed to do: stop legitimate telemarketers from calling you, without risking a fine.
What it can NOT do is stop criminals calling from VOIP or other easily faked phone numbers, most likely from India or the like, well out of the reach of any US law or agency. If you're on the DNC list you can be assured that ANY telemarketer who calls you is likely a criminal scammer.
If you want to learn more you can go to the DNC web site and learn what it's really all about.
Actually the DNC is meant for the guidance of law-abiding telemarketers. It does not stop scofflaws or criminals. Many of these calls originate in India and Pakistan and as such beyond the reach of US law.