I have a bookmark for https://www.getcreditcardnumbers.com/ - I happily give them all the credit card numbers they want (the ones from that site pass the checksum, but of course isn’t valid in combination with a made up expiry and CVC).
After a couple card numbers fail, they cuss me out, sometimes threaten my life, and never call again.
My theory is they get flagged by their payment processor if they submit many bogus credit card numbers.
It’s about a 10-minute investment once a month. Less time than I used to spend answering and hanging up on spam calls.
(1) Usually they think my name is the guy who had my phone number almost 10 years ago. I “correct” them to a fake name, but it shows their record keeping is not good enough to track anyone down.
(2) If they tried to follow through on the death threat, they’d have a hard time getting a visa with “need to kill citizen” as the justification.
Early on, I had a temporary bump in calls after doing this. If you stick with it for a few weeks, eventually you’ll get on enough “real” do-not-call lists that the calls fade away.
My record is a call of around 14 hours. The autodialer called me after 10:00pm (supposedly illegal here), and there were no operators to take the call. I left my phone charging with the call active, and went to sleep, since the caller pays the call. Kept the call until I needed to go out, and I like to think that even if the call wasn't expensive because it was bulk price, maybe having a line busy helped slow down spam for others.
I don't do that anymore because spam calls have multiplied, it would mean answering more spam than I'd like.
My assumption is that they have some sort of CMS software and that it costs money to call. If you don't answer - they'll keep trying you. But if you do answer and costs them money - they'll put you in the 'do not call' list.
Just my guess - but so far it has worked for me personally.