It will probably be like a Bluetooth headset with a phone built in. You just speak into it to tell it who to call, and be told who is calling whenever you receive a call.
Maybe solar powered, or powered by electrical impulses of the body. Right now we have watches, etc, that can read the electrical signals of the body, so they might find a way to harness this energy source. Something like how humans are used as batteries in the Matrix.
On another note, when it comes to 'powering the system' as in The Matrix, people are already being used as batteries by the capitalist corporations as advertising, films, other social media, the tablet - which is basically a mass consumer-device designed to generate profit and reduce thought - is already determining people's every move and impulse for the purpose of maintaining elite control and economic ascendency. How else do you think Facebook, amongst others, can be worth as much as it is?
Human volition is now centrally located outside of your immediate physical vicinity.
So the Matrix is here.