Once an earthbound spirit is self-aware, that is they know that they are dead, they will begin to learn to manipulate objects and people. This, of course, can be good and bad. Electronics are especially easy for spirits to manipulate, and if a naughty spirit wanted to scare you out of the house, what better way than for you to receive a phone call from "the devil." Of course, you are talking about the 90's and the days before most had caller ID or could do reverse phone # searches online. Living people have been hacking phones for years. Either way, you definitely had some type of prankster on your hands, living or dead.