On some aircraft, such as the S-76 it can cause spurious baggage bay smoke / fire warnings.
In our aircraft, carrying a switched on phone causes heavy interference on intercom, whether in use or not, so we can always tell if a passenger hasn't complied with our directive in the safety brief to turn them off.
Also, regarding the use of a hand-held mobile phone by a pilot in flight, the UK ANO states that a hand held mike is prohibited from use in PT aircraft during takeoff and landing, or inside CAS below FL150, even for communication inside the aircraft. I'm sure if you use a h/h mobile phone it comes under the same remit, so expect a visit from your local CAA legal branch man if you get found out, possibly for endangering the aircraft if you have a silly turn like the no gear down man.