What to check

At the telephone check the DC voltage between the 'L' (-) terminal and the 'F' (+) terminal, it should be between 6-12v DC.

Check that the wire to 'L' is making a sound connection and that the terminal screw is tight.

'L' in the telephone connects through to 'D' In the door panel. The 'L' wire to this phone may pass through another terminal point in a junction box or another phone. Check both ends of the wire have a sound connection.

Corrective Action

Ensure the 'L' wire is connected soundly at both ends.