You also ask about mobile phones. The 'cellular' system, which mobile phones use, depends upon your phone making it's position known to transmitters in the area. (When somebody calls you, the call isn't routed to every transmitter in the country. It's only sent to the transmitters near to you). It does this by periodically sending out a signal which means 'Here I am'. (You've probably left your phone next to a radio or hi-fi and heard the sound coming through the speakers).
When a local transmitter picks up this signal, it sends an acknowledgement back to your phone. If no acknowledgement is received, the phone tries another signal but with greater power. If your phone still doesn't get an acknowledgement, it increases the power and tries again. This means that a phone which is nowhere near a transmitter (e.g. when it's several miles up in the air, on board a plane) will be periodically pumping out high-powered radio signals (which can interfere with the electronics on board an aircraft) while trying, unsuccessfully, to contact a transmitter.
Note that the phone doesn't have to be making or receiving calls for the above situation to occur. It's sufficient that the phone is switched on. For this reason, mobile phones must always be switched off, when on an aircraft. Simply having a phoned switched on, without actually using it, is a criminal offence, punishable by 5 years imprisonment.
Chris