What to check

Check all the connections in the telephone are correct and in accordance with the appropriate wiring diagram. 

Trace the cable from the phone back to its next connection. This may be in a junction box, the power supply, or possibly the door panel.

Corrective Action

The simplest way to determine the "colour code" that has been used is to open another phone on the system and note the colours used. Note the wire to A (or A2) is the "call wire" and the colour of that wire changes dependent on which door button it is connected to. 

However, if you don't have access to another phone on the system you can determine the correct connections by following these steps;

1. Identify the "F" wire by measuring for DC voltage between each conductor trying each in turn.  The "F" wire will have a stable DC+ reading of around 6-12 volts on a multimeter. The F wire is common for speech both ways and when linked with B and G is the common for all phone functions.
   
   
2. To find the "L" wire (speech from the door) and the "P" wire (speech to the door) both will read a stable DC- reading of around 6-12 volts relative to the "F" wire. To determine which is "P" or "L" so try each on the "L" terminal and when you can hear audio from the door that is the "L" wire.
   
   
3. To identify the lock release wire, terminal "1" (terminal "3" on old model phones) find which wire has an AC voltage relative to the "F" wire.
   
   
4. Lastly, to find your "call wire"  "A" or "A2", you need to have the door panel button pressed, and find which wire has AC relative to "F".