Originally Posted by diburning

Also, the reason why cell phones are not allowed during take-off or landings is because phones emit a signal during the transmission of text messages that can interfere with speakers (or in this case, the speakers in the pilots' headsets). Don't believe me? Put your phone next to a stereo that is powered on, as close to the magnet as possible, and play something that's quiet. Send or receive a text message, and you'll hear the noise loud and clear.

Now imagine hearing that noise in your headset when you're trying to talk to air traffic control.