Several years ago my wife started getting prank calls from an invisible number. When she answered, the other party kept silent. No heavy breathing or background noise, just plain silence. She had a clue that it might have been some student(s) from the school where she worked as a teacher, but she had no evidence.
So one afternoon her cell phone rang once again, not showing the caller's number, and I picked up the phone. In a formal, emphasized way used in taped messages I said: "The number you are calling to is under Police surveillance. Please hold, your call will be connected in a moment." That was the last prank call from the silent caller.
