i think cell phone noise would be rather rare. i've open taped and stealthed on many many occasions and not once have i ever had cell phone noise in a recording...not even when the phone was on and in my pocket just inches from the recorder.
also, the first time i saw the mt in stealthing action there was a girl who was holding up her cell half the night so her friends could hear and i don't here any cell phone noise on the recording anywhere.