My Roaming data was off!

[Anonymous]

I have just returned from the US with my family.  Before I went having been stung by texts in the past I did the following:

My Vodafone iPhone: Turned on roaming and braced myself for the £5 a day fee!

Sons and Wifes Vodafone iPhone - Turned off data roaming and mobile data - Only used via wifi in the house

Daughters O2 iPhone - Turned off data roaming and mobile data - Only used via wifi in the house

On my return to the UK I received a bill from O2 for over £250, with over £200 being for SMS sent by my daughter only when she was on wifi as she didn't have her phone on her any other times.  Every time I gave her her phone I ensured roaming and data were off and I checked the phone several times while she was on it and every time it was wifi connected and data off.  Other than a valid £5 roaming charge for one day for my wife, there were no charges for my wife or son Vodafone who sent texts from wifi, just like my daughter.

I have called O2 and they are saying the charges are valid and they are doing nothing about it.  They also asked me why I didn't call them before I went to get advice!    Finally, I was told the charges for using data while using wifi and with roaming and data turned off 'may be unethical' but they are valid and need to be paid!  

Any advice would be greatly appreciated as this feels very unfair!

[MI5]

Check and see if wifi assist is turned on in her phone settings?

[Anonymous]

Thank you! Yes wifi assist is on (as it is on all 4 phones). Would this override mobile data and roaming being set to off? Also, the wifi was the same for my wife and son and they had no charges and the wifi in the Airbnb we stayed at were better and faster than the UK....

Just checked the Apple site and in response to lots of criticism and questions they confirmed:   Wi-Fi Assist will not automatically switch to cellular if you're data roaming.

[MI5 · Accepted solution]

It's not supposed to work when roaming at all but I'm struggling to think of any other ways you could have legitimately incurred any charges.
Are the charges only for sending text messages?
To be charged for these, they must have actually been sent over the mobile network, or were they iMessages?