It usually only happens if someone is on your radio frequency. This is not a runaway so much as interference. The receiver doesn't know which signal to receive and reacts to both sources. With nitro vehicles, the receiver battery will go dead and the car will do whatever it was doing when it lost the signal.
It is unlikely that a cell phone would acuse trouble. To avoid interference, make sure noone is running on the same frequency and you shouldn't have any trouble. Also, make sure you antenna wire isn't touching the aluminum or carbon fiber on your chassis.