BILLED FOR DATA NOT USED

In November I upgraded my contract. While at the store, I was told they cannot upgrade as I have international roaming activated. I asked them to cancel it as it is not needed. The consultant confirmed it has been cancelled and proceeded with my upgrade. It was done and I left with my new phone. Then in December I went overseas. But before I left I went to the same MTN store and purchased a prepaid SIM to use while travelling. Needless to say it never worked so I bought a local SIM which I used throughout my trip. On my return on the 21st December, I put my contract SIM back into my phone while I was at the airport. I had no signal but I used the airport wifi while waiting. Then boarded the flight and landed back in SA on the 22nd. 2 days later I get a sms saying I used 7708MB of data and my billing is about R7200.I called MTN to query this as I have not used my phone. To date this amount still reflects on my statement and has not been corrected. I have logged several calls and even requested the url statement which has no data for the 21st of December but does have usage from 22nd December which is minimal. Is this how MTN rips off their long standing (11 yrs) custome