I'd play the Nickelodeon theme song on repeat at 180 decibels for as long as the caller is on the line.
If they hang up, a voicemail will be sent to them that contains... you guessed it, the Nickelodeon theme song at 180 dB. It will be played only once in this case.
Truly what an ingenious way to fry your phone's earpiece/speaker in addition to your ears and those of everyone around you within at least a kilometer of distance, all by listening to a too loud overamplified distorted rendition of those 5 notes known to be associated with Nickelodeon. I'd go to jail for causing such a disaster (both for "ear terrorism" AND copyright infringement).