As I wrote back in 2001, I still respond to telemarketers with, "Please put me on your do-not-call list," and I hang up.
But I think I'm going to occasionally try something new. When they pause, I'll say "I don't do business with lawbreakers."
I hope they say, "Excuse me?" so that I will have a chance to reply, "You broke the law when you called this number which is on the do-not-call list."
As they sometimes do, maybe they will then say, "But we spoke last year, and you asked me to call you back later."
That will give me the perfect opportunity to say, "I also don't do business with liars," and I'll hang up.