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Our family plan is unlimited talk, text and shared 10GB of data among four phone lines. I remind everyone on the plan to turn off cellular data when they're using WI-FI (free internet) so we know for sure we are not using cellular data and that will prevent from overcharges on our bill.
If you have an Apple iPhone, go to: Settings > Cellular > Cellular Data. When it is green, that means you have cellular data on. When it does not show green, that means you are turning off cellular data and using WI-FI.
One time I was using WI-FI at home, and the the WI-FI was experiencing problems so automatically I was being switched from WI-FI TO LTE (Data) without knowing! Luckily, I did not go over the data plan or else it would be a hefty charge!
Wi-Fi Assist on your iPhone will detect a slow network, and automatically switch to cellular data. You will never know this is happening because the phone will not alert you.
The way to avoid big phone bills and overage charges is to disable this feature, which comes enabled when you buy the phone. Go to Settings, open the Cellular Menu, scroll down to the bottom and disable Wi-Fi assist.
Source: I just read this on clarkhoward.com, and have done this.
