From the 'Life of a Field Telephonist' a series of 6 postcards by Fritz Lindshoeft, 1917.
Field telephones were first used in the First World War to direct troops. They replaced flag signals and the telegraph as an efficient means of communication. The first field telephones had a wind-up generator, used to power the telephone's ringer and batteries to send the call, and call the manually operated telephone central.
Images Courtesy of Queen Mary University of London Archives.