WTF?! If you’re traveling abroad, always be aware of any potential roaming fees when using your phone. As one San Jose family found out, the costly charges can quickly mount up: soon after they came home from a vacation in Vietnam, T-Mobile sent them a bill for $13,470.

Vivian Chung and her children had just come back from their annual trip to Vietnam, where they celebrated her mother’s 84th birthday, when they received the bill for roaming and internet charges. What was even more surprising for the family was that their iPhones were set to airplane mode as a way of avoiding these fees.

The massive bill was reportedly incurred by son Nicholas during a half-hour period while flying over Vietnam. He admitted to playing games during this time but said the mobile apps were offline. However, as KGO reports, some apps that don’t require internet connections still use data to update software, send ads, or refresh social media. The chess game Nicholas was playing is one such app.

“When cellular data is on, apps and services use your cellular connection when Wi-Fi isn't available. As a result, you might be charged for using certain features and services with cellular data,” notes Apple.

T-Mobile didn’t explain why the bill was so high, but it did say Vietnam wasn’t included in its free roaming plan, though that’s now changed.

After appealing to T-Mobile, it agreed to reduce the charges to $3800. That’s still a huge amount, of course, but when ABC7 also contacted the company, it wiped the entire bill.

Remember, if you’re traveling somewhere that isn’t part of your provider’s free roaming plan, the best way to avoid fees is to turn off a phone’s cellular data.