I went down the states for 1 day from 2pm to 11pm. And when I saw my bill, I was charged for 2 days! I called customer service and was told I used my data around 12:04am but we were already home around that time. She said there's nothing she can do coz the system registered it. But I think that was so unfair to be charged for something I did not use. They should be able to do something for their system's error.
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Sorry to hear you've incurred unexpected roaming charges. I understand you might not have intentionally used roaming. However, if the usage is on the system then your phone would have used services without your knowledge. The most likely culprit would be data usage. Even if you had disabled data and roaming data, some phones consider their system data separate from user data and allow its usage despite those settings. You might consider enabling Airplane or Flight mode on your phone to prevent future unintentional usage.
Hope this helps 🎉