My teen daughter gets calls like that all the time.. she gets the strangest look on her face and freaks out and throws me the phone. I chew them a new one.
They called me today. Usually I don't answer then block them but decided to answer but not say anything. Just got dead air then hang up. Next time I might just play my music as loud as I can in their ear, see how they like being pranked. Hehehe :)