A phone. Field phones were very much a thing during WWI and WWII, with point to point wiring laid down very soon after gaining control over the territory, or even before that.
Re:Tricks of Trench Warfare (2020)
by DyslexicAtheist on Monday June 14, 2021 @09:03AM
(#27502025)
also carrier pigeons: https://en.wikipedia.org/wiki/War_pigeon
In the pre-transistor times, radios were heavy and used a lot of energy. Even in WWII, field radios weighed about 35-40 pounds. A soldier with a radio could not carry a lot of other equipment.
https://en.wikipedia.org/wiki/SCR-300
Vacuum tubes are a clumsy technology.
did they have radios? Or would they send a runner?