411A - a very valid point regarding proper equipment and cable shielding.
But were your 7 passengers using analogue or digital phones? Because it is the digital pulses generated from phones which seems to cause the most - and the weirdest - problems. I have a cheap small colour TV in the kitchen - and if my digital cellphone rings, the TV will switch itself on and display odd screen captions which don't appear anywhere in the instruction leaflet! Digital phones can also cause strange buzzing sounds over car stereo systems unless a proper hands-free kit is fitted.....
I had one of the first GSM phones which I used in an 'airliner type' flight drck on the ground on many occasions with never any problems. But it was an old wires and clockwork dials flight deck - not a full-glass. Quite probably the problems caused by digital phones interfering with some aircraft systems simply aren't fully known - hence the ban on phones being switched on in airliners.
I flew a little PA28 with FM-immune King radios the other day - one of the passengers (despite my instruction) had forgotten to switch of his GSM phone and the frequent 'hand shake' pulses could be heard interfering with thee intercomm. Nothing else was affected - but the problem posed by passenger phones shouldn't be underestimated.