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Hello idembovska and welcome to the forum!
That does sound a bit odd as you have Data Test Drive on your account. It sounds like one for the eForum Team to look at, as it is account specific, so please just sit tight and one of them will be along soon enough to drop you details of how to get in touch.
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I'll pop you a PM so we can see if anything is amiss for you. Just follow the directions within.
You can find your PM inbox here. If you can't access this link, please ensure that the box next to "Turn on private messages" is ticked on this page, and let us know if you need any further assistance.
Keep in mind that messaging and mobile data are displayed together under the usage breakdown and on the itemised bill - it's the icon next to them that shows what type of usage each record is - so if you have used any chargeable text services these may be mistaken for data sessions. We'll be able to see for sure and get to the bottom of things when we get your details.