What to check

On the door panel speaker unit, measure the voltage between the '+' terminal and the 'D' terminal which should read between 6-12v DC

If there is no voltage between '+' and 'D', disconnect the wire to 'D' and re-measure. If you now have a voltage, this suggests there is a short between the 'E' wire (marked 'L' in the telephone) to the '+' wire.

Corrective Action

Trace the short circuit. This could be anywhere on the system but it is quite common for it to be in a phone that has been disturbed by decorators or builders.

Change the 'D to L' wire for a spare conductors if required.

Additional notes

A method to determine where on a cable a short circuit might be is to measure the loop resistance on the suspect conductors.

CW1308 telephone cable typically has a 100 meter loop resistance of 30 ohms. E.g. a resistance of 3 ohms suggests a short is 10 meters or less away.