That's futuristic enough, but several companies, including NTT DoCoMo and South Korea's Samsung Electronics have already developed those. Most of this interaction takes place via a sensor mounted on the underside of the user's wrist. The sensor includes a speaker and a microphone, the former pointing inward and touching the skin while the latter is mounted facing outward. When a call is being made the user inserts their finger into their ear. Sound travels via bone and cartilage from the wrist to the finger and then vibrates through to the ear, where it is picked up, explained Masaaki Fukumoto, a senior research engineer at NTT DoCoMo's Media Computing Lab and the developer behind the system.
Callers sound a little distant, and the volume is low, but it definitely works.
The microphone is more conventional and simply picks up the voice of the person speaking because, with their finger in their ear, the underside of the wrist is naturally close to the mouth. The final innovation comes in control of the telephone hook. A call can be started or stopped by tapping your second finger and thumb together with different rhythms signifying either the beginning of a call or hanging up. The same system cannot yet be used to dial yet - the prototype doesn't even have a keypad, but in the future, the user could tap the second finger and thumb in certain patterns to control dialing and other functions. Fukumoto, who is part of a six-member team that works on wearable technology at the cellphone operator's research and development lab in Yokosuka near Tokyo, cautions that the cellphone might never come to market but that some of the technology being worked on now may well make it into cellular telephones of the future.