We wouldn't know, unless she is using You Tube or watching films etc. However, as we have no access to any account you should message O2 on Facebook (https://o2uk.co/O2CFB) , Twitter ( https://o2uk.co/O2CTW) or Instagram (https://o2uk.co/O2CIG) , they should be able to help you with this.
*The Game Is On*
What type of phone does she have @Joanna1 ?
Most have a setting to use data if the WiFi connection is not great - something like WiFi Assist. This will mean that the phone is using data even if connected to WiFi.
Please note, this is not customer services and we cannot access your account. Do not publish personal details (email, phone number, bank account).
Link to our guide on how to contact them can be found here
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