Again ShyTorque, I do totally agree that there is no point in having cellphones working on airplanes.
But, on the other hand, you are operating one of the few types on which, according to your experience, you can reliably reproduce the effect of a cellphone interference to an airplane system. It may seem a little thing to you but despite all the discussion here there is very little literature on the subject.
I think this is a precious opportunity to correct a potentially dangerous situation that exist in your airplane.
You know that telling people to do or not do something is never enough. Passengers can be distracted, act in hurry, or for whatever reason still leave a phone on in cabin or in the baggage holder, in perfectly good faith. And then you are confronted to the awkard situation of making a landing call because the fire alarm goes off.
I understand the reluctance by the owner/operators of doing anything that is not absolutely necessary, but consider the huge amount of safety and securty procedures that they already undertake, also for legal protection reasons.
The solution is not said to be costly or complicated, RF interferences can be often solved with a bit of intuition, experience, and a 10c ferrite bead strategically placed.