reachme

Rather than have multiple phone numbers, you would have just one number, or username really: say, "Fred". Calls forward to whatever device is near you at the time, and based on your preferences.

Better than a mobile phone, you are free to use what's in your environment. You go home, and calls route to your full-size, hi fidelity handset. In the car, calls go to your in-car speakerphone. While jogging, calls reach you on your ipod (iphone).

The change of routing could be manual: ie you press a button on the device, or automatic: a chip your carry identifies you and broadcasts your presence.

Imagine how liberating and powerful this could be. Mobile phones really suck: the batteries die, they break, get lost / stolen. SIM cards make them somewhat dynamic, but really why should you be tied to one piece of hardware at a time? You can check email from multiple devices / locations, right? Why not phone calls? Imagine going into a hotel, the phone rings and it's your husband checking to make sure you arrived okay. You didn't even have to tell him which hotel or room you were staying in. And no fumbling for your cell phone (and did the battery die on the long flight..?)

To boot, you could provide context-aware info to the caller. For instance, if calls are going to your car, the caller might be alerted that you are driving and to "keep it short..". Taking it one step further, for trusted callers you could enable an in-car webcam, so they can peer into your environment & know not to distract you when traffic is hectic.

You could also screen calls/callers based on context. For instance, only family members while jogging.

This wouldn't really require a major change in infrastructure, mostly just new devices. The cell networks would have to allow two-way data connections.