There is reasonable doubt that Cell Phone Frequency Tx can affect Communication & Navigation Equipment/Instruments on Board an Airliner.But it has not been proved as yet.
But since the Rule Forbids its use.We stick to the Rule.
I usually switch it off after making one final call to family/friends after boarding the aircraft. I switch it on only after the aircraft arrives at the gate.
On this note, in Malaysia, they ask you to switch it off before entering the aircraft and switch it back on only after entering the terminal building.
So, it is different rules for different countries I guess!