My favorites:
1. When they ask to speak "to the person who makes decisions on purchasing" or whatever, I say "I'll put you through to the finance department" make a beeping noise, and then in exactly the same voice say "Hello, Human Resources, how may I help" they then usually say "Sorry I wanted the fiance department" ..I say I'll put them through, make another crappy beeping noise and so on....till they get bored !
2. After the initial spiel say "I'm glad you called, there is something I want you to do for me, have you got a pen..."yes".(they usually sound excited, thinking they have a lead)...please write this down..."DON'T EVER CALL THIS NUMBER EVER AGAIN...GOT THAT?"