Using someone's sim card, any chance of consequences? ATNT?

I bought an IPhone 5 off craigslist from a BUY/SELL/TRADE stand at the mall. Of course they must have bought the phone from someone else before they sold it to me, so the sim inside most likely isn't theirs. I discover the sim card when i got home, it has unlimited call, text, 4g service on it. My girlfriend doesn't have mobile service so i decided to let her use it. Believing that the service would cancel within weeks, because the original sim card owner would cancel before they sold the phone and the service would remain for the month. My girlfriend has been using it for a over a month, would there be any consequences in the future if she continues to use it? Is Atnt able to track or anything?
I don't understand why this person would not cancel their service… Yet.

Yes, you can get in trouble, and will when the other person gets the phone bill. All they have to do to find you is use the phone's internal GPS system. What is wrong with your moral compass? You had to have known what you doing was wrong, and your girlfriend has to know it wrong,. And both of you deserve what is going to happen to you. No excuses, no rationalizations, makes what you did ok.

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