My recollection is CNN on air reported that the phone was not logged into the cellular network when the plane took off. They speculated that something must have caused him or someone to switch it on. They also said they were attempting to clarify an ambiguity in that their source confirmed that no phone call was made but would not or could not confirm or deny if a text was sent or attempted to be sent. No further information seems to have come to light.
That fits with my recollection. Can't find anything more definitive about this. One of many oddities that seems to have quietly disappeared from the public domain.