Sir may wish to peruse the user guide linked therefrom
http://www.britishtelephones.com/useguide/ambassa1.htm
40 Bell Off.
41 Bell On.
Dial 9 to use the first available free line or:
81 First Line.
82 Second Line.
Intercom: Dial 0 followed by the terminal number.
External extension adapters are not compatible with MF signalling, if MF is required a dual signalling telephone is required and must be set to LD, and only switched to MF once exchange dial tone is received.