TURN OFF YOUR CELL PHONE WHEN YOU BOARDED the PLANE

There are very good reasons why the aircrew always remind remind passenger that it is strictly forbidden to use cell phones in the cabin of the plane.
Modern aircraft depend greatly on radio waves to perform numerous functions, such as communication with the control tower, navigation, and regulation of cabin atmosphere. Radio wave interference from a cell phone can seriously disturb these functions.
You may not be aware that even when your cell phone is on standby it still sends out an electromagnetic signal that notifies tje cellular network's computer that the phone is active and can be contacted. This signal becomes stronger when the transmitter at the base terminal station (BTS) communicates with the cell phone to send a voice call or short message(SMS).
Actually, once the aircraft has taken off and approaches its cruising altitude, your cell phone will not work anyway, because the plane is too far from the BTS . Furthermore, the plane moves so fast that once the phone's presence has been detected and registered by a cell in the cellular network, the phone is no longer within that cell's range. Yet if your cell phone is active, it is continually emitting electromagnetic signals that could seriously disrupt the aviation equippment, thereby endangering the lives of hundreds of passengers
so for your own safety and that of your fellow passengers, you must turn off your cell phone asa soon as you enter the plane. If you have a phone in your carry-on luggage, check to make sure that it is turned completely off

source : garudamagazine