This indicator gets bridged across (in parallel with) an analog POTS line or station port. The LED will light when Neon Message Waiting Voltage is being sent.
IMPORTANT NOTE: A real Neon Lamp needs about 80VDC (minimum) to glow. This solid state LED device will trigger at a much lower 50VDC used on some modern phone systems. This is an electronic device (doesn't use neon gas), and works on neon message waiting that's 50 to 140VDC.
MW1 Neon Message Waiting Indicator Specs: