Since a cell phone is not physically connected to the telephone network and lines through a telephone wire then there's no chance of getting struck by lightning through your cell phone.
Using your cell phone during a lightning storm or thunderstorm is perfectly safe.
Now with the old cell phones they had an external antenna which maybe the lightning could strike when it was extended in the air but with the new cell phones without the external antenna it's safe to use the cell phone during the thunderstorm.