The intercom here is based on technology, still functioning, that it nearly 90 years old, the Tele F field telephones.
Parts of the phones are described in Herbert and Proctor, the books from which Atkinson's Telephony was derived.
Advances in communications have obsoleted my repertoire of telephony jokes, eg, ...
Q. What should you do if a herd of elephants is charging towards you?
A. Make a trunk call and reverse the charge!
Q. Now Miss, can you use a Diktaphone?
A. Certainly not. I use a pencil just like everyone else!