Although telephones had been invented more than 40 years before, World War I was the first war where field telephones where widely used. Armies used them to communicate across the battle landscapes, from one camp to another. Telephones carried across battlefields needed to be portable, like this one, used by Germans during the war. 

Inside this oak box is the telephone receiver, the phone cord, a circuit to allow someone to talk on four different lines, and a diagram of the wiring. The inside of the front has metal tags with the alphabet. Each letter has a German word next to it. When someone needed to say letters over the phone, or spell something out, they would use the words to make sure that the listener understood which letter was being said. These are often called "call letters" or "radio alphabets." Other armies used call letters during the war. In Great Britain, many of the letters corresponded to men's names: C for Charlie, D for Don, E for Edward, F for Freddie, G for George, H for Harry. Other letters related to British government: K for King, L for London, Q for Queen. The United States used different words (Cast, Dog, Easy, Fox), and by 1957, many world organizations adopted the same list of call letters.