See
https://www.britishtelephones.com/er...etin/11_15.htm for the jack/junction box wiring.
The index to all Sam Hallas' Q duagrams is here
http://www.samhallas.co.uk/repositor...ir=q_diagrams/
For the extension-extension on overpress, there's a circuit whereby if the
called extension is engaged on an exchange line, the buzzer at the
calling station will operate to give an engaged signal. So for
ordinary working there must be some way on the
called station that the calling current is routed via an exchange line key to the buzzer, and for
engaged working the current must be routed back onto a home line for the
calling station.