I saw this on a TED talk a few months ago, but I'll paraphrase:
With VOIP, the phone calls they make are essentially free from anywhere in the world. So, the thing that costs them money is the people they employ at these call centres. If you waste their time, you waste their money; when one of these jokers calls me, I play very dumb for as long as I can keep it up (assuming I don't have something going on the stove).
WASTE THEIR TIME. If you hang up immediately, they move on. If they're on the phone with a person who's dicking them around for three minutes, that's three minutes they're not spending scamming someone else. I found that telling them over and over again things like, "No, I don't think so" or, "No, that doesn't sound right," really, REALLY frustrates them (and is pretty amusing to boot).
Same goes for "Canada Revenue Agency." They will only ever send you a letter; they'll never call you.