Despite the fact everyone on the plane already has their damned phone on, they are totally safe in call mode as well. If a cellphone can screw up the instrumentation of your aircraft, there's something wrong with the aircraft, not the passengers.
It CAN screw up the instrumentation. It won't affect flight controls or engines, however, it can mess up with the radios.
You know sometimes you hear some noise in your speakers or headphones when you are about to receive a call. Pilot headsets are no different and will pickup cell phone radio noise exactly the same way. In another instance, a cell phone external battery pack leaked enough radio noise to open the squelch of a nearby portable air band radio. There are all personal experiences. And if you don't think it is a serious problem, radio interference was involved in the deadliest plane accident (see http://en.wikipedia.org/wiki/T... ). Beside communication, ILS systems (required for low visibility landing) are very sensitive instruments and I don't know how robust they are to cell phone radiation.
Well, I hope that airliners are somehow less sensitive to radio interference that the small aircrafts I pilot but still, I once boarded a plane where the pilot made an announcement along the line of "someone is using a cell phone, I won't take off before it is switched off". Meaning that it did have a noticeable effect.