use the owd un, when they ring, say who am i speaking too, how do you know mr/mrs ******, then say when did ya last see them, n you are Detective Inspector ******, and have just found their body, please give me yer address, etc etc, ya can have bags of fun whilst they are then crapping themselves.
p.s. they will usually say they don't know the person, then say well why have you rung to speak to them when you dont know em.