Mtn doesn't want to cancel my contract

I had a contract with mtn, I paid for 24 months and I sent am email to cancel my contract in March. I was told that I owe R1012 because I have been using free data and I told them that I didn't not get free data. The other consultant said I missed payment last year for 3 months and I sent them all my proof of payment. Now the other consultant said I owe this money because I never canceled my contract in time, and I have all the reference numbers when I was trying to cancel my contract and no one help me.