Better safe than sorry...several years ago I had one of the presidential limos on board our trash hauler...prior to takeoff both hsi started spinning, I finally asked the loadmaster if anything was going on in the back..the secret service boys had turned on the com radios in the limo, once turned off everything returned to normal. More recently with a tower controller in the jumpseat on an A320 we were delayed for thunderstorms, both engines shutdown...apu running. The controller asked if she could use her cellphone...our procedures allow for cellphone use on the ground prior to takeoff and after landing. I had no problem with it since we were not going anywhere for awhile. She made her call, and we could hear her over ground control...I asked her to turn it off than redo it just to see if it would happen again...it did! Sorry, I think we all know from flying aircraft long enough...that just because the tech says it can not happen, doesn't mean that it won't. If your need for being in touch is so critical...don't go.