Few devices know more personal details about people than the smartphones in their pockets: phone numbers, current location, often the owner's real name — even a unique ID number that can never be changed or turned off. These phones don't keep secrets. They are sharing this personal data widely and regularly, a Wall Street Journal investigation has found. The findings reveal the intrusive effort by online-tracking companies to gather personal data about people in order to flesh out detailed dossiers on them. Smartphone users are all but powerless to limit the tracking. With few exceptions, app users can't "opt out" of phone tracking.