Quick Google searches indicate about three watts. To a first approximation, you are not concave when talking on a cell phone, so figure at most 1.5W travelling in your direction. Humans are definitely not opaque to radio, but let's suppose I am. Personally, I weigh 90kg, so for me it's something like 15mW per kg at most, for the small handful of minutes per day I spend on the cell phone.

My level of concern about this could not be lower.