One of my club members had his phone in his pocket whilst flying, the phone rang and his model immediately went out of control and crashed . Subsequent investigation of the remains found that some of the memory settings ( in the transmitter ) were different to those used while the model flew properly.
Further investigation by another member proved that mobile phones could produce changes to transmitter settings. Synthesised transmitters are known to be affected.
Consequently in our club mobile phones must be switched off near the flight line. This is also a recommendation of the British Model Flying Association ( the governing body in Britain ).
Read about it at [link=http://www.bmfa.org/news/bulletin/sb6-mobiles.htm]www.bmfa.org/news/bulletins/sb6-mobiles.html[/link]