[Milu]

Ask for their HOME number so you can ring them back.

Ask them to hold the line while you get the householder and keep them waiting until they hang up.

Anwere their questions with questions that have nothing to do with the subjecyt matter, then ignore the answer and ask more irrelevant questions, always give them time to answer.

Blow a referees whistle down the phone at them

Ask them lots of personal questions under the guise of selling them insurance.

when you understand it's a telemarketer stop answering their questions and just leave long silences. Arrange for a visit for when you're not home, ideally when they'll get caught in rush hour traffic, or to an obscure address.

All very childish, but so satisfying after a bad day at work.