The telephone was not used only for voice messages: a buzzer system sent messages over the lines using morse code. Direct two-way voice communication over the telephone obviously had the advantage of immediacy, but the one-way message allowed for encoding – an important consideration when eavesdropping on telephone lines was easy.
The Field Service Pocket Book gives examples of two ciphers for encoding messages. The sliding alphabet and the Playfair cipher were described as “simple and comparatively secure.”(1)
Strict instructions were laid down for enciphered or coded messages, such as reminders never to write the message in clear and in code on the same sheet of paper, and to destroy all paper used in the process of encoding or decoding. And signallers were enjoined not to guess encoding or decoding, but always to work “in a mechanical manner.” (1)
(1) Field Service Pocket Book. 1914. 212
The telephone pictured is the D Mark III, which became the standard model of field telephone with a buzzer. The image © IWM (COM 581) is from the Imperial War Museum.