Its hard to believe that anyone who has flown a plane much has not heard the interference of a cellphone handshake (repeated creaking sound) over the RT or intercom.
It is equally hard to believe that anyone with enough technical knowledge to fly an aeroplane cannot understand that if the RF from a phone can induce a signal like that in a comms system it can do the same in any other electronic system - eg a control system.
As said above, there need to be too many 100% protection levels in too many different aspects of systems to assume immunity, and we cannot possibly predict how a signal from a mobile device in an infinite number of positions and orientations might be reflected and concentrated by structures and fittings into a vulnerable spot.