I never answer any number I don't know, but it has not stopped me from getting deluged with calls to my cell phone and my landline. If you have an answering machine or service where they can leave messages, their system recognizes that it is a valid phone number. So not answering doesn't do you any good, because your answering machine did answer. I believe they already know they have valid numbers because they get them from valid databases.
However, I do think answering just to argue with them is a waste of time and likely gives them some personal satisfaction. I don't answer and once I see it's an unknown number I check it here online to be sure it's not someone I'd want to call back. Otherwise, I ignore them and periodically delete all messages from my answering machine.
Criminal minds, such as the people who do this kind of thing, have no morals or ethics or shame. Nothing you say to them is going to penetrate their depth of worthlessness.