Having the telephone conversation interfered without notice is very uncomfortable. So that the user is not a victim of this type of listening, through the extension phone, we describe a simple circuit that can monitor the phone line when the devices are in use. Using two bicolor LEDs interconnected to the device line, all according to the diagrams presented. How it works: when one of the handsets has its handset off the hook, a chain will circulate through the LEDs causing them to light up. This will indicate that the phone line is being used. Using the device connected to the TM-2 socket, the LED (VD) lights up for the extension phone and LED (Vm) for the main phone. If the LED (Vm) lights up when one of the devices is being used, this is a sign that there is a bug, or the handset of the other device is off the hook.
