Poor Service and Incorrect Debit

I have had a contract for 2 years with Cell C. I was told by the store that I would have to give 30 days notice to advise that i did not want to continue. I called on the 1st of March to give notice on a contract ending the 4th of April. I advised the call agent that I would not want to be charged for 4 days only in April and she agreed that Cell C could end my contract at the end of March. I have now been told that my notice was 30 days for 30 days. My account has now been debited at the end of April with not the original R399.00 but R605.00. I am very upset by this and will be taking Cell C to the Ombudsman.