I felt like messing with the telemarketers a few times.
My favorite is to let them get to a point in their shtick and then interrupt and ask a totally unrelated question. I usually do this in a real - oh shucks, I don't know nothin bout nothin tone. Then when they answer me, I'll interrupt again and claim to have forgotten what they wanted from me in the first place. So far 20 Minutes is my record. I almost felt sorry for that poor gal. I don't think she'll make it in sales.
Another one is to pretend that it is a friend calling and talking to him/her in "code."
Of course I can answer in German - but I consider that cheating.
...well, that was before the QB - now I just let the answering machine pick up.