I just wanted to ask whether it is possible to voluntarily discharge a mobile phone's battery and transfer it onto another battery?
It is possible to tranfer the energy stored in one batery into another. However, this is not as simple as just connecting the two batteries together. If the batteries are of the same type, then that will cause each to be about equally charged.
Assuming batteries of the same type, the voltage of the one that is to be fully charged will end up higher then the one that is to be drained. It would take a active circuit to transfer the energy. That is possible, but there are various inefficiencies along the way. The circuit won't be 100% efficient, and the receiving battery will lose some energy in storing and retreiving. As a rough guess, you'd probably get only 1/2 or less of the energy from the first battery eventually to come out the second at best.