Whopity is correct.
However the variable RF output cuts both ways, and aboard an aircraft at say 2000ft the received signal will be very poor, so the phone will wind its power output to maximum.
That's why phone battery life is usually much worse in the countryside
Also, most of the battery life improvements of the last decade or so have been achieved by discontinuous transmission, whereby the transmitter is cycled on/off. This also dramatically reduces the power radiated into one's brain.
They have also stretched out the interval between an inactive phone transmitting to contact the base station(s). It is now IIRC about 10 minutes. So if you have a phone just lying there, it will not radiate anything (of significant power) except for a second or two every 10 mins.
But despite all that, the
peak power which a phone can put out is still as high as ever - 400mW I think, though the figure varies according to which frequency it is on (900/1800 etc).
If you want to see if a phone is interfering with avionics, you will need to do it in a low signal area, and actually make a call with it (or receive a call).
Much more common cases of avionics interference are VHF-GPS issues e.g. the 11th or 13th subharmonic of 1575MHz clobbering the GPS receiver. Try transmitting on 121.15MHz or around there and watch the GPS signal levels.