contract with MTN for 24 months ended

I am having a contract with MTN for 24 months, and my last debit was in 26 May 2015. I called MTN call centre to cancel my contract in March and the explained to me that for them to be able to cancel my contract I must not have airtime in my phone and I must call in May to cancel this contract. 26 May 2015 I call again to cancel my contract they told me that they will do it and gave me a Ref number (***), the lady that was assisting me told me that she will cancel my contract and transfer my number to prepaid which she didn't do. Then I call again on the 02nd of June now being assisted by a guy he change my payment method from being debited to cash payer and explaining that he will change my number to prepaid. Now I receive airtime and I call again today the lady call Busi assist me and she's saying I'm owing MTN R145.00 that I must pay before they can cancel my contract account she gave me the ref ( ***) and said I must go pay this amount. <br> What i fail to understand is my contract was for 24 months and now I will be paying for more then 24 months. *** <br>