4.7t is pretty strong. It isn't a cover detector as it's mainly iPhones I've seen affected and it lasts long after the phone is removed from the field. However this may be what has caused the effect on some of the other phones I've seen exposed. I'll do some experimenting next time it occurs. GE and Seimens seem to have an array of phones that engineers are equipped with and they are happy to expose them to high fields.