Is it possible to convert voice signal into electrical energy in mobile phones?

In principle, yes, but you wouldn't want to wait around to do the job that way.

Human ears are very sensitive, and the sounds around you carry very little energy

compared to the energy used by common electrical gadgets. One or two watts of

pure tone from a loudspeaker will drive you out of the living room, and the "ear buds"

are blasting no more than a few milliwatts of sound power into your ear canals.

You could hang a microphone out of your window, and use the sounds of traffic

as you drive. It might charge your cellphone in a couple of weeks ... if you leave

the phone turned off and don't use it.