My groundschool instructor told me about a friend of his in his new Cirrus that had a switched on mobile in the centre armrest/cubbyhole (like a Ford Mondeo I assume.) Apparently the interference gets stronger the further away you get from the source and the phone caused a problem when it came to disconnect the autopilot.
I think it had caused real damage to the system and a CB or two had to be pulled in order to disengage it. Not exactly first hand as stipulated but I have no reason to doubt its authenticity.