If in the plane was in the middle of the ocean, the of course. But by the time the families realized something was amiss an started checking the phones it could have been over land according to the most recent speculation.
If the plane never made it to land, certainly you are correct.
But then we would know where they were when their phones reconnected to the network. That would be a major clue in the story, eclipsing the recent satellite link news. But we don't know, and this little factoid simply isn't a fact.