Wikipedia
A sound-powered telephone is a communication device that allows users to talk to each other with the use of a handset, similar to a conventional telephone, but without the use of external power. This technology has been used since at least 1944 for both routine and emergency communication on ships to allow communication between key locations on a vessel even if power, including batteries, is no longer available. A sound-powered phone circuit can have two or more stations on the same circuit. The circuit is always live, thus a user simply begins speaking rather than dialing another station. Sound-powered telephones are not normally connected to a telephone exchange.
Usage examples of "sound-powered telephone".
The enlisted man who always sat on the edge Of the hole wearing a sound-powered telephone headset that enabled him to talk to the men in the catapult machinery spaces was already in his place, 'Staring aft at the two Planes on the cats.
Four enlisted men wearing sound-powered telephone headsets stood behind the transparent board and kept its information current by writing backwards on the board with yellow greasepencils.