Pretty much same thing happend to me. I called tmo and pretty much acted really pissed off(as I was) but I was still KIND, just annoyed by their service.
I told them there was NO WAY I used up my five gigs and that it must be a flaw/glitch in the system. Obviously told them I'm a good customer and that being throttled is a game changer and that if they did not reset my data I would probably switch carriers the same day. I said i would also file a complaint with the BBB so that I didn't have to pay a cancellation fee(yes that works I and some friends have done it before ).
And voila they reset my data with hardly any trouble at all
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