What to check

A 'D to E' (also known as an 'L to P') short circuit can be anywhere on the system.

At the door panel disconnect the wires connected to the 'D' and 'E' terminals and measure the resistance between the two wires. If all phones are 'on-hook' it should be open circuit. If a handset is off-hook a multi-meterr will give a reading of about 300-500 ohms. A short circuit will be much less, at only the loop resistance of the cable.

If a handset 'off-hook' is suspected, ring each phone in turn from the door panel. If a phone is 'off-hook' the sound of the ringer will be heard at the door as you call that phone.

Corrective Action

Replace any handsets that are 'off-hook'

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.

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 metre loop resistance of 30 ohms. E.g. a resistance of 3 ohms suggests a short is 10 metres or less away.

Consider changing the suspect conductors for spares if there are any.