I find it entertaining to turn into an old pervert with an old man voice who can't hear so well and always misinterprets something they to something sexual.
They hang up pretty quickly... "whaat? whaat you say? you'd like to fondle my balls? well I guess that would be alright. would you really like that?"
Throwing in a lisp is also great too... but yea usually, i don't answer.. what i do though is google the numbers and put the numbers in facebook to see if anythign pops up.
that homicide bit above is pretty good lol