IM ENOUGH WITH MTN CONTRACT

I am fed up with mtn when my contract ended i have been trying to call them to cancel my contract, i did not get help. my credit score was messed up because of mtn. I have to settle the account with an amount of R2400, after that i struggled with them to close my account again and struggle again to get a paid up letter. They send me a paid letter after a month. This month I receive an invoice saying im owing R287.98. How can they bill my account if its closed and issued a paid letter? I phoned mtn last week and also 30 min ago they gave me this nr *** it says the nr is incorrect!