1) someone calling and pretending to be someone they're not -- usually for comedic purposes
2) someone attempting to harass the person they're calling
3) a goofy radio bit that's been around for decades (see #1)
In the days of land lines and no caller ID (the early-1990s), prank phone calls happened quite frequently at our house. Full disclosure: prank phone calls often emanated from our home as well. Whether it was someone calling and hanging up as soon as we answered or a late night voice on the line informing us there was a gas leak at a nearby home, the volume of incoming prank calls increased during the summer of 1994. Seeing as my brother was away for the summer, we didn't think it was someone from his circle. As for me, I couldn't name any enemies that doubled as suspects which made the mystery all that more frustrating.
Finally fed up, my mom called the phone company (it was named Ameritech in those days). In order for Ameritech to tap our phones, we needed to file a police report that detailed the harassment. The two of us made the trek to our local police department where the policeman turned to me and asked what was now a familiar refrain, "Any problems at school?".
"None," I responded. Truly. I couldn't name any enemies. In fact, I attended summer school that year and didn't have the time or energy to have any enemies (that I knew of).
After sending our police report to Ameritech, they tapped our phones for exactly four weeks. During that time, they instructed us to keep a log of prank phone calls. For exactly 28 days, zero prank phone calls rolled into our home. None. On day 29, when the tap expired, the calls resumed. Was this some sort of inside job? I don't know, but it was maddening. The phone calls continued: someone would call and then wouldn't speak after our initial "Hello?". This person eventually got a bold and would tap on the phone receiver in an almost Morse Code. During one of the prank calls, I grabbed the letter from Ameritech and read it to the moron on the other line. With great smugness, I let them know our line was tapped and they would be caught (even though I knew the time expired on the tap). Finally, a muffled female voice appeared on the other end that said something along the lines of "Gosh, Todd, we were just playing around..." followed by the line going dead. The voice wasn't familiar, but they clearly knew me. Who the hell could this be?
Months later, someone from my journalism class revealed the pranksters were two girls I knew. Neither girl disliked me but, for whatever reason, got the itch to prank me that summer.