RGA telephonists in a captured German dugout, near Langemarck, 21 August 1917. Communications was the weak link in battle planning in World War I. All the information these signalers were passing from forward observers back to batteries could be interrupted by a single shell that cut the lines. Wireless telegraphy technology required very large antennae, and until late 1917 the crystals were imprecise enough that W/T sets would interfere with each other if they were closer than 3,000 yards, about two miles. In the absence of huge quantities of carrier pigeons, messenger dogs, or human couriers, telephony was the best alternative, even with its attendant problems.
Source: IWM photo Q2750.