I once was told that it has to do with frequency modulation. While the cellphone & aircraft systems use different frequency ranges to communicate with distant stations (cell phone tower vs ILS antenna, for example), the aircraft electronics and the pax electronic devices could modulate the external used frequency into a smaller shared internal frequency on various electronic components. I'm not an electronic engineer so i don't know anything about this, but apparently it can come to interference with engines if, for example, you are in an aircraft with tail-mounted engines and a pax in the very rear seat uses his cellphone, which could interfere with the electronics that control the engine controls - since they are fully electronic these days.
Always wondered if there's much truth to that or if that is just hearsay.