Mobile phones vary the amount of power used to transmit, based on the signal strength in the area in which they are operating. Generally this means the further you are away from the nearest mobile phone mast, the higher the power output of the cellphone in order to keep the contact going.
In the case of a cellphone in an aircraft, the higher you climb the higher the power output of the cellphone until when you have no useful signal from land based aerials the cellphone is blasting away at maximum output and does so frequently as it tries to locate a new connection. It is at this time that the cellphone is the most dangerous when in an aircraft with loads of electronically controlled critical equipment such as FADEC etc.
Recent ideas are to place a mobile phone "mast" inside the aircraft which is connected to a rebroadcaster outside the aircraft. The interior "mast" only needs to transmit a fairly weak signal to facilitate connection with the passengers cellphones which should also only need to transmit a fairly weak signal to maintain communication. Not sure if this has already been put into practise, but it is certainly being thought of. It also means that those annoying ringtones will invade your sleep on long haul flights together with the familiar conversations like "hello mate. I'm on a plane over Paris"
T4