Although bell buzzer codes are used less widely these days, they're included here as they're a favourite question topic for assessors and examiners.
Code Meaning
1 Stop
1-2 Close power-operated doors
2 Ready to start
2-2 Do not open doors (driver and guard to speak to one another)
3 Set back
3-1 Lock central door locking
3-2-1 Testing doors
3-3 Guard required by driver, or guard or driver to speak on the telephone
3-3-1 Release central door locking
4 Slow down
6 Draw forward
9 Police assistance required
Module TW1 Section 9 states:
You must make sure that all codes are made carefully, clearly and distinctly, with pauses clearly marked and acknowledged by repetition (except for code ‘3-2-1’).
If you receive a code ‘9’, you must get police assistance at the next suitable stopping point. You must arrange this by telling the signaller in the quickest way possible.
You must use the cab-to-cab telephone only for essential conversations about the working of the train.
You must not use the cab-to-cab telephone instead of the bell or buzzer codes to control movements of trains, unless your train operating company instructions allow you to do so when the bell or buzzer is defective.