There is, on the operator's switchboard, a supervising lamp associated with the calling plug as well as with the receiving plug. When a subscriber calls the operator, the supervising lamp under the receiving plug lights, as before noted. When the operator "plugs in" and connects herself with the calling subscriber, that lamp is extinguished. When she "plugs in" at the calling hole, the second supervising lamp lights, and remains lighted until the party called answers. So long as these two lights are extinguished, the operator knows that the subscribers are using the line. When the subscribers replace their receivers on the hooks, the lamps relight.