What to check

The Monitor when called from the door or the ON button pressed sends a pulse down the SIG wire to start the central power supply timer, which then supplies 20v to monitors for about 1 minute before timing out.

At the door, ringing any button should start the power supply timer which then powers the camera and the lamp above it as well as sending 20v to the monitors.

If the SIG wire to the monitor is open circuit, then pressing the button will NOT start the timer and the lamp above the camera will NOT come ON.

If ringing a different button, starts the timer, then the monitor should work until the power supply 'times-out', this implies that the SIG wire to this phone is 'open circuit'.

To check the SIG wire at the monitor you should have about 12v DC voltage between SIG and "-" ,

In case there is a fault with the monitor not sending the pulse down the SIG wire, you can manual simulate this by shorting SIG to "-" and then pressing ON.

Corrective Action

Use a spare conductor if required.