What is a unique feature of sound-powered telephones?

Sound-powered telephones are distinct in that they operate without batteries or external power sources. This feature is significant because these devices rely on the sound of the user's voice to generate electrical signals through a built-in generator, making them exceptionally reliable in situations where conventional power sources may fail.

This capability is especially valuable in environments like ships or remote locations, where access to electricity can be limited. It allows for communication to occur under various conditions, including emergencies, without the need for rechargeable batteries or power outlets. Thus, the fundamental operation of sound-powered telephones sets them apart from other communication devices reliant on external power.