Kyocera is working on a speakerless smartphone that uses bone conduction rather than traditional speakers. It's ideal for use in noisy environments, because it doesn't waste energy trying to transmit sound waves through the air to your ears.
Kyocera's prototype smartphone has a vibrating screen; when you press the screen against your cheek, ear or even muffs covering your ear, the screen's vibrations send the sound waves along your facial tissues and bones to your inner ear and thence to your brain.
(Kyocera prototype speakerless smartphone)
The first person to describe a device using bone conduction to improve sound transmission was sf's own Hugo Gernsback, who filed a patent for an "Acoustic Apparatus" in 1923; his patent was granted by the US Patent Office in 1924.
This invention relates to acoustical instruments and the important objects of the invention are to provide simple and practical means by which hearing may be effected by sound vibrations transmitted directly to the osseous tissue of the body. A particular purpose is to provide such means in the form of a small, compact and handy instrument which can be easily carried about and used without attracting undue attention.
Science fiction fans also recall the audio relay from Robert Heinlein's 1951 novel The Puppet Masters; essentially, it is an implanted cell phone.
Bone conduction is an idea whose time has come! See these additional stories for more prototypes and more information: