Nowadays, with the internet, and calls being much more traceable than they used to be, the perverts have many other ways of frightening people, but I remember that it was quite a scary thing in the 90s and earlier; there were documentaries about it, and in one book of safety stories for children (stranger danger, etc.), there was one story about a child who answered the phone to one of these callers, and the story was full of advice on what to do about it, including the now discredited advice of blowing a whistle down the phone.
Scam phone calls tend to be the unwanted ones now, but I'm just wondering if the lewd ones do happen nowadays, and if it's something we should warn our children about, as well as all the many, many other things.