telkom refusing to cancel my contract even after i have given my notice

so my contract ended january and then last month i went to telkom store to cancel my contract they gave me a case number ***6. today i called to find out if they had cancelled it because i saw an invoice that they still charging me for data and minutes and the agent told me they have not cancelled and she does not understand how. now am given another case number ***9 . i dont understand why they not cancelling this because i dont want their services any longer