The circuit receives its power from a 15-V wall adapter (not shown). The circuit takes advantage of the fact that th
e phone line voltage drops from 48 to 10 V when an extension is taken off the hook. When the voltage on a line drops, the optoisolator/coupler is turned off so that the inputs to the line-1 hexinverters (U2 pins 1, 3, and 5) float high. The corresponding outputs (U2 pins 2, 4, and 6) go low and light the line-1 LEDs.