Looked at making a carrier change and had my Note 3 unlocked by AT&T in September. Normal usage for a single month is under 2GB for a family plan shared with the wife. We've not gone over the 2GB plan in over 20 months of service with AT&T. For the past two months in a row we've gone over our plans even when being out of the country with our phones not connected to the AT&T services.

Month 1: Switched to a T-Mobile SIM card prior to departing the country and was within our normal usage. Gone for 4 full days. Returned from the trip and placed my AT&T SIM card into the phone. Within 1 hour I we received a message stating our allotted 2GB of data was almost completely exhausted. Ended up going over for the month by 1MB.

Month 2: Same situation as Month 1 but out of the country for a full week. Within 1 hour received a similar message.

When I spoke with AT&T and looked online, it showed that I had used 64,000KB of data at 3PM yesterday which is impossible as my flight hadn't landed. Assumed this was looking in arrears for a 3 hour period but AT&T couldn't confirm.

How does AT&T track the usage for a period? Does it take the information directly from the phone or does it take from the system? What could have changed in the following 2 months to drive me over by so much?