Hello again ppruners, once again I find myself the horses mouth re: the mobile phone setting off the igniters.
This is not a crewroom myth "It was me!"
Aircraft concerned was actually an AS355F1, and happened exactly as Bladecrack described. Made me jump to be sure and was repeatable i.e got someone to ring the phone while I stood on the skid step alongside the engine bay - sure enough each time it went off, so did the igniters.
Our company policy has not been shaped by that incident, but by a report (I will find it and post it)
I try never to anger the Gods of aviation by doing anything that MIGHT just be a problem - so it's phones off in my helicopter.