It's the longstanding myth about mobile phones on planes. Switch them off when flying or it could interfere with the plane. 

Could leaving your phone switched on actually bring the whole plane down? No. Of course it couldn't. 

However it can affect communications between the pilot and air traffic control. The interference usually amounts to nothing more than the noise that can be heard when a phone rings too close to a speaker. However instead of coming out of a speaker it's coming through the pilot's headsets. 

There has been only one reported incident of someone's phone interrupting a radio call between the pilot and air traffic control which could have potentially caused danger.

So although it can happen, the incidents of it happening are quite rare. For every fifty flights, it might occur once or twice. However it's still unknown if this is the result of the effects being naturally minimal or from people generally being good at remembering to switch off their phones. 

Via Independent UK