Depends. There is lots of irrational fear with this and as far as I know, no solid proof for harm, but there is extra electricity flowing in your head, because of the transmission of your phone. I don't like talking on the phone that much anyway, so I do not care so much, but if I would talk for hours, I would use a headset (also for ergonomic reasons).
A phone battery is 3.8 volt. No worry, right?
Good point. I don't know much about these things. Are the radio transmissions from a phone stronger? I would expect so, but don't know.