There is, however, the possibility that the people who got tumors were somehow susceptible to them (i.e. would have likely gotten them anyway) and that the cell

There is, however, the possibility that the people who got tumors were somehow susceptible to them (i.e. would have likely gotten them anyway) and that the cellphone use merely influenced which side of the brain the tumor formed. Or perhaps there is some other factor (handedness, the side you sleep on, the side of the car you usually sit in, etc.) that both biases cancer to one side of the head and influences which hand you use to talk on the cell phone.