Users such as Gizmodo writer Chris Mills have seen a jump in data use since downloading the new system. Mills said his data usage has risen by about one-third, and he suspects that WiFi Assist is the culprit:
It’s impossible to say if that extra usage is directly related to Wi-Fi Assist, but I have my suspicions. On the iPhone 6s that I’ve only been using for three days, my data usage is at 950MB; half of that is from Netflix, which I make certain to never use when I’m on the go. In fact, the only time I’ve used it in the past couple days was at home, using what I thought was Wi-Fi.
From what Mills reports, it seems as though the problems start when users think they're somewhere with a good WiFi connection, but their phones beg to differ — leading to bill shock. Users concerned about that can turn the feature off near the bottom of the "Cellular" section of their Settings app.
So can you avoid surprises in the future? The iPhone, just like many other phones, has a built-in counter that helps you keep track of what you're using. Apple's customer service forums are a little split on how accurate it is; some say it hews closely to what they see on their bills, while others say it is a little off. It's probably best to look at it as an estimate rather than a definitive tally and to check in with your carrier to see how much the company is billing you.
That said, it can be a handy snapshot. To find it, head to that same Cellular menu in your Settings app. Scroll down and you should see a section called "Cellular Data Usage" that gives you an estimate of how much data you've used since you last reset that counter. If you've never used this tool before, you've probably never reset it. To do so, head to the very bottom of the Cellular menu and hit "Reset Statistics."
You'll have to reset the counter manually — maybe when your bill comes each month.
You can also use this part of the Settings menu to keep certain apps from using cellular data at all. This menu lists all your apps, and you can decide, service by service, which apps you want to allow to use cellular data. (It's also handy for international travel, when you may be working on a really limited plan.) Turning a few of these off — streaming video apps such as Netflix, for example — can cut down on your data usage. And if you find yourself really craving some "House of Cards" on the bus, you can easily flip that switch back on.