My friend does Fonejacker to them brilliantly, the I ate him/her always works well. My mum just puts the phone on it's side and lets them talk for ages before they realise there's no one listening.
Go tell them "Yeah yeah I'm reeeaally interested, I've got someone important at the door, can I just leave the phone for a tick while I speak to them??"
"Yeah sure"
go do something for 15 min and come back and yell profanity if they're still there.
*ring, ring* Dad: Hello Telemarketer: Hello my name is Valarie and i was wondering if i could interest you in............. Dad: Well Valarie, that sounds lovely but my wife doesn't let me talk to other women. *click*
i got a call once from some credit card company asking if i wanted to expand to some other card or something, and i told them about how i don't use credit cards ever since the bank heist i pulled off a few years back. the line just went silent so i hung up lol