[jm951]

Motion-
A cell phone in the air is going to light up way more than 10 towers. It's that line of sight thing since a ground based phone can see maybe 3 towers max. From 30k feet the line of sight is huge. We used to chat with a guy in the ANG who would take up a 2 watt radio on training missions. From southern GA, he could peg our local repeater 5x5 from an altitude of about 12k feet. IIRC the transmitters on Voyager and some of the other space missions aren't very powerful and yet we can still hear them way out into the solar system.

As for avionics interference, all radio antennae have a degree of what can best be described as a "dirty" signal. In other words, there is some RF transmitted off freq. It occurs on wavelengths that occur at a harmonic point to the antenna and wavelength in question. Depending on what freq your cell is operating at, you could well be interfering with the avionics to some degree something that might be undesirable from a pilot's pov.