When I get a sales call, I quietly put down the phone while they're talking and let them talk.
Then they start with, 'Hello? Hello? Hello?'
It totally confuses them and throws them off script since there's no one there to respond to their questions.
One thing that is fun to do is pretend it’s someone you know. Say something like “Ahhhh, it’s you! Drunk again? Call me back when ya sober up!” An Irish brogue helps.