I did phone pranks in my teen years with a friend of mine. I had a little tape recorder that I would hook up to the phone so we could play back the best ones for more laughs later.
My favorite gag was I'd find two people in the phone book that lived near each other. Then I'd call one and tell them I was with a private investigation firm and was wondering if they had any information on the other person. Of course, I'd call the other one too and ask the same about the first person. I was amazed at how easily they would go on about whatever "dirt" they knew about the other. And, if they didn't know anything they'd just tell stuff that they "think they heard" about them, etc. They were never skeptical of the call -- they were just totally into digging up dirt on the other person if they could.