Data problem

[Wyer]

My son has 4gb data a month as of Friday he had 2.56 GB left but then this morning got a text from o2 to say he has run out of data but phone has been off or he has been at home with WiFi so what is happening?

[MI5]

@Wyer 

At a guess, WiFi is weak or has dropped out so phone has been using mobile data. 
If he has WiFi assist turned on it should be disabled and data turned off when connected to WiFi. 

[Wyer]

Thanks MI5

No WiFi assist is off so just have to wait and try sort with o2

[madasaf1sh]

@Wyer

Depending on the handset, you should be able to go into the setting menu, and have a look at which application is using mobile data in the background.

On Android you can also go into the Connection Setting and check that the Mobile Data Only Apps box is unticked

On IOS you need to go Settings and Mobile Data to see the same

[Wyer]

Thanks for answering my question
But in this case phone was off due to flat battery and when back on had nothing running.