"Hello. This is not a telemarketer. You have been selected-"
"Hallo? Entschuldigung, ich spreche deutsch nicht!"

That's not a bad idea. I'll have to do it next time. Well, next time I get a human, anyway. Usually the telemarketers I deal with are all robots, which makes it easy to hang up, but you can't really mess with them. Course, if they happen to actually speak German, the poor grammar and the fact that I just said "I don't speak German" would give it away...