With a telephone is on hook, the voltage across tip and ring will be approximately 50 VDC. When the phone is lifted off hook, the tip to ring voltage will drop to a lower voltage.

With the phone on hook, the diode bridge provides 40 VDC. Current from the phone line, limited by the 10K resistor, is sufficient to light the LED inside the optoisolator and turn on the transistor. The output voltage at pin 5 of the optocoupler will be near 0. 2 VDC. When a phone is lifted off hook, the drop in voltage will cause the internal LED to go out. The transistor will turn off allowing the voltage at pin 5 to rise. This signals the CAT controller not to process any autopatch requests. If this circuit causes a continuous off-hook condition, increase the value of the 10K ohm resistor. Note: This circuit is not FCC Part 68 certified.