There were reports from Apple's iPhone users getting frustrated with data overage charges that have been caused by activating the Wi-Fi Assist feature. It has caused higher phone bills to subscribers.

The iOS9's Wi-Fi Assist automatically switch the internet connection to cellular data if the Wi-Fi network is too slow.  

For example, if you're using Safari with a poor Wi-Fi connection and a web page doesn't load, Wi-Fi Assist will switch the connection to cellular data so that web page continues to load. Wi-Fi Assist can be used with most apps like Safari, Mail, Maps, and more.

But the feature switches seamlessly from a Wi-Fi connection to mobile data connection, so the users sometimes are not aware that it's happening. As a result, the users experiencing that the phone is using more data than they think.

Responding to complaints, Apple posted a new support page on the feature that explain how it works and that Wi-Fi Assist will not have much effect on phone bills.  The support page states that Wi-Fi Assist will not automatically switch to cellular data if the user is on data roaming.

The support page also explains that Wi-Fi Assist only works for apps running in the foreground and does not work on background downloading contents.

Sixcolors agree, saying Wi-Fi assist doesn't work with some third-party apps  that stream audio or video, or download attachments like email app.

The Wi-Fi Assist is not the only feature that can be responsible for the data issue. Data charges can also be caused by a subscriber apps Settings. Users can turn off the "Cellular Data" option for apps that consume a lot of data like Netflix, Amazon Video or Hulu.

The users can also save their data usage by disabling the background refresh on apps that are not used regularly.

However, if the users would like to play it safe, they can deactivate Wi-Fi Assist  on Apple iOS 9 by going to Settings > Cellular > Wi-Fi Assist.  The users can also search "Wi-Fi Assist" by using the search bar under Settings.

But if you want to keep the feature on but just want to be sure that you're not using up all of your allowance, the iPhone offers the option of checking up on how much data it has used.

You can see the data usage in Settings > Mobile Data Usage. You can set the Mobile Data Usage for the current period, and you can choose to reset the settings every month.