For wire breaks on airplanes in production we use a Time Domain Reflectometer (TDR) made by HP. With the right equipment it will only take minutes to determine the distance from the wire end to the break. But we know where the wire routes and its total length. Finding a cable underground will be different. I believe the phone companys use a device that follows a signal given off of the wire until you get to the break then it disappers or changes. Most wire breaks occur where stress or damage occurs. If the line was nicked when installed it would get noisy as ground water makes contact with the exposed conductor, then eventually the exposed conductor will erode away by electrolisys. Any ground work near the line should be suspect. My experience has been that wire breaks of this nature occur at the ends or near, or any splices in between. Another approach would be to go to the half way point and tap in to determine which direction has the break then go half way in that direction.
