What to check

Check that the voltage at any phone between terminals 'F'(+) and 'L'(-) is between 6 - 12v DC

The 'L' wire in the phone is for incoming speech (from the door) and connects through to the 'D' terminal in the door panel amplifier. You should get the same voltage at door between the '+ ' terminal and the 'D' terminal

If there is no voltage on the 'D - L' wire it would suggest it is open circuit either because of a poor termination or a break in the wire itself.

Check terminations in the door panel, power supply and the main junction box for loose connections.

Corrective Action

Remake or tighten loose or suspect terminations.

Consider using spare conductors if required.