MTN CHARGING INCORRECT AMOUNT ON ACCOUNT

MY SAMSUNG S4 MINI TOP UP CONTRACT WAS TRANSFERRED FROM NASHUA TO MTN IN DECEMBER LAST YEAR. I STARTED TO NOTICE THAT MY BILLS WERE EXCEEDING THE AMOUNT THAT WAS AGREED UPON ON THE CONTRACT. AFTER FURTHER SCRUTINY, I DISCOVERED THAT I WAS BEING CHARGED EXTRA FOR BIS WHEN THE CONTRACT IS NOT EVEN FOR A BLACKBERRY!!!! HOW IS THIS LOGICAL IN ANY WAY???? I HAVE SPENT HOURS ON THE PHONE TRYING TO SORT THIS OUT (BECAUSE NO ONE ANSWERS AT THE CALL CENTRE AND IF THEY DO, THEY HAVE NO IDEA HOW TO HELP). ALL THEY DO IS REASSURE ME THAT THEY WILL REMOVE THE INCORRECT DATA CHARGES AND GIVE ME ANOTHER REFERENCE NUMBER BUT WHEN I SEE THE BILL AT THE END OF THE MONTH, IT IS STILL INCORRECT!!! I HAVE ALSO TRIED GOING INTO AN MTN STORE TO SORT OUT THIS MATTER BUT I WAS UNSUCCESSFUL. I AM SO SICK OF HAVING TO WASTE MY TIME SORTING SOMETHING OUT THAT SHOULD NOT BE THIS DIFFICULT!! SOMEONE NEEDS TO SORT THIS OUT AND I WANT A REFUND OF THE MONEY THAT WAS INCORRECTLY DEBITED FROM MY ACCOUNT!!