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1. The phone can determine its location via GPS, cell tower, and/or wifi positioning.
2. The phone has Internet access. (Cell or wifi.)
As you might suspect, the south Indian Ocean doesn't have many cell towers or wifi hotspots. GPS signals don't penetrate water, nor do the frequencies radiated by cell phones. Even if you had an unobtainium phone that survived the crash and depth, it couldn't be tracked.