What form of communication was used during wwi?

The radio : Radio made its debut years before World War I — it was often used by ships transmitting messages via Morse code, and in 1912, operators on the Titanic depended on radio to communicate with other ships and with onshore radio stations.

The biggest improvements radio offered over message systems like Morse code were the speed and accuracy afforded by the use of voice communication. Advances in radio technology such as oscillators, amplifiers and the electron tube made reliable voice communication possible.

The "wireless" (as early radio was sometimes called) quickly proved invaluable to wartime efforts: Radio operators with portable transmitters, for instance, were able to warn soldiers of an attack of poisonous gas, giving them time to don their gas masks.

The German army used radio transmissions to guide dirigibles making bombing runs, and radio proved indispensible to communication with another recent development: airplanes.
(source : https://www.livescience.com/45641-science-of-world-war-i-communications.html )