Robo calls I don't mind. We still have a landline and it's nice to be able to give that number to people so the Cell numbers don't get harassed.
That said, I have sometimes just started swearing. Not necessarily at them (although one or two felt my wrath). My wife says I could get in trouble for it, but my stance is, I didn't invite them to call me. If it's not someone I do business with, they are a free target.