Originally Posted by ferrismo

As posted previously there is a small possibility that a phone can interfere with the avionics. If everyone had their phone on and transmitting i.e sending a txt, in a call, data session or even registering to cell this would greatly increase the chances of causing a problem, during the critical stage of take off and landing. Once you up in the air the risk is reduced, radio contact is not essential to keep the plane flying, but on the ground the plane needs to be in constant contact with the control tower.
You turn your engine off when you fill up at a petrol station right? Even though the risk of causing an explosion is very small if it was left running, so why do you turn it off then???
The answer is the same as why you are asked to turn your phone off on a plane.