If you have installed on your iPhone, the new iOS 9, then it can happen that your mobile data volume is consumed despite Wi-Fi is activated. This means that your iPhone with iOS 9 always has access to the mobile data network of your mobile operator.
The reason is as follows:
In iOS 9 the option “Wi-Fi Assist” is automatically activated. This option is intended to offset a slow Wi-Fi connection by the simultaneous use of mobile data. Is your Internet at home slower than Apple addresses it, then the mobile data is automatically activated in order to obtain a certain speed limit.
That is why we recommend you to disable Wi-Fi Assist on the iPhone with iOS 9 to rest your mobile data volume and to refrain from unwanted costs. To disable Wi-Fi-Assist on the iPhone with iOS 9, proceed as follows:
1. Open Settings --> Cellular
2. Disable Wi-Fi Assist using the controller
Should you own a iPhone4, then you cannot find the function because wireless Assist is only possible with an iPhone supporting 4G / LTE connections.
You know now why your iPhone with iOS 9 consumes more data volume as it was the case in the previous iOS versions.