Would it not be great to have a small software on phone which would auto reply to the ignored number the owners unavailability to answer their call. It may even give out a signal as in the phone is reachable! So next time I want to ignore a particular number... I just add him to ignore list.

Sound is a form of energy. And phones use cell/battery to run. They store electrical energy. While we talk over the phone we give out sound energy. We can have a small device which converts sound energy to electrical energy and charges the cellphone batteries while we speak. It can even be charged in a theatre, pub, discos, traffic signals basically anywhere with a decibel level.