why cellphones should be turned off in the plane

Based on personal experience I had not accidentally turn on the phone in the airplane, not deliberate but because I forgot, thank goodness there is no sms or voice calls.
 
If the phone rang I definitely will be in trouble with the flight attendants. That's why I became curious about exactly why the phone must be switched off before flight,  and why flight attendant has great emphasis on this thing  in a safety demonstration before the plane departed.
 
So this is the fact that I get from various sources, with this fact i hope we all realize the dangerous of communication signals within the aircraft safety.

Modern aircraft depend greatly on radio waves to perform numerous fuction such as comunication with the control tower, and navigation.

Not a lot of people aware about the danger of turning on their cellphone inside a plane, the fact is even when your cellphones is on a silent/standby mode it still sends out an electromagnetic signal that notifies the cellular network's computer that the phone is active and can be contacted. The signal becomes stronger when the transmiter at the Base Terminal Station (BTS) comunicates with the phone sending a voice call or short message.

Actually, once the aircraft has taken off and approaches its cruising altitude your cellphone will not work anyway, because the planes is too far from the BTS. Furthermore the planes moves so fast that once the phone's presence has been detected and registered in the cellular network, the phones is no longer within the network range.

Yet if your cellphone is active, it is continually emitting electromagnetic signal that could seriously disturb the aviation equipment, thereby endangering the lives of hundreds of passengers.

So for your own safety and that of your fellow passenger, you must turn off your cellphone as soon as you enter the plane.