Since they have been mentioned in this thread, I make this post about them, rather than the Garmin type GPS receivers that started the topic.
In Brunei, the mobile phone system is crude, to say the least. The handsets are always making search transmissions and at full power. When they can't get a response they ramp up the power and try again. At high power outputs the transmission power is sufficient to break through on my tape player. Yes, you read that correctly, the tape player. It produces a deafening burst of sound so loud that it makes an unsuspecting passenger jump.
Now, in an aircraft, if you leave a mobile phone switched on, it will still search for a pole to link up with. Even over the ocean. Transmission power that is sufficient to be picked up and amplified by the pick-up in a tape player will definitely cause interference to aircraft electronic systems. Unless and until they fit mobile phone transponders into aircraft to allow the phones to connect on low power, let there be no confusion. No-one can safely leave a mobile phone switched on in an aircraft.