There is simply not enough energy in the microwave emissions from a cell phone to cause any damage to human tissues. It might make you a little warmer, that's it.
I rarely use the phone directly to talk.. When on a call I am either in my car with the car's built-in bluetooth or use my earpiece.
Wasn't there a warning about having a cell phone in your pocket because it could decrease sperm count or something like that?
Just use the Apple Earphones with the Mic or a Bluetooth if you really want to reduce the chance of getting a tumour.