I'm going through practically the same issue with my iPhone 6, mine turned out to be the sim card but they still won't credit my account for the overage amount. Granted mine isn't quite as much as yours but I had my phone cut off temporarily due to an outstanding balance but they cut it back on one day due to the credits supposed to hit my account the next day. That was like almost 3weeks ago on that
Good luck with issue. I know your frustration.
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