Here's What Happens If You Don’t Put Your Phone In Flight Mode On A Plane

Some persons just like to be 'stubborn' for whatever reason, by refusing to put their phones off or on the required Flight Mode while on a plane. The reason flight attendants insist you put off your phone or at least leave it in flight mode is to prevent the pilot getting distracted by interference noises being emitted from your phones.
You may have heard the ‘bzzt bzzt bzzt’ crackling noises from some radios when a phone is nearby. That sound is never pleasant, especially to a radio station if you are calling in.

That is what some pilots are subjected to if you don't at least put your phone in Flight Mode. It's not like the plane would crash, but it causes distractions for the Pilot who should have concentration.