Microphone for doors - Karma GM 20 P

It is intended for use in public shops such as post offices, banks, information desks, etc.. It consists of a 38 cm long swan neck microphone dedicated to the internal operator and a second small microphone to be applied to the outside of the door with call button.
Both have a small speaker to listen to the other voice through the safety glass.
It is powered by a small external transformer including