Again a Better responce from EE who seem to have identified the issue and provided a solution:-
I've spoken to EE again and I think this is something people are experiencing who use IPhones
spoke to a helpful lady who said she's had the same issue. Go to Settings - Messages - Send & Receive. ensure that the phone number is the only thing ticked and not the email address
go in to your messages and delete the texts so it doesnt try and default back.
It seems that on an IPhone when it makes an iMessage it is interpreted as an MMS so I have done what EE Technical support has advised as BT Technical support were not interested and will see if this solves the issue!
So EE refund the MMS costs and provide a technical solution - BT Customer service and Technical support say go away its your issue have to love BT
On an iPhone, the best thing to do is:
1. Settings > Messages > turn off “Send as SMS” - this means an iMessage will always send as an iMessage, rather than falling back to an SMS or MMS (which otherwise happens when you or the recipient has a poor data connection).
2. Settings > Messages > turn off “MMS messaging” for good measure, so you can’t inadvertently send anyone a multimedia message. NB: you can’t receive MMS either if you turn this setting off, but who really wants to get charged for sending you a picture message these days anyway!?