If the telemarketer/scammer is already breaking the law, being on that list isn’t going to help you. It also won’t keep debt collectors from calling if they think their target (or someone who knows that person) is there. We get multiple calls a day for my sister-in-law because she used to be a neighbor and because of the relationship; databases will show that stuff to debt collectors.
It worked for me one time - an ex roomie with a debt collector. I told them that the call was being recorded, and that not only was my phone a cell phone, it was on the do not call registry and that while my ex roomie may have some sort of tenuous relationship with the debt collector and hence the do not call registry would not apply to her, she no longer lived with me and I had no such relationship and the next time they called I would sue them for harassment. Stopped the debt collector from calling me about her again. Obviously this would not work with a debt collector with one of your debts … although I should call a bud and ask about it. If you posit that you have a business relationship with AT&S, obviously they can call and dunn you, but you have no actual relationship with a random debt collector who bought your debt from AT&T, you never made a contract with that debt collector so theoretically the debt collector shouldn’t be able to call you if you are on the do not call registry.