OVER-CHARGING AFTER THE CONTRACT HAS ENDED

I took a contract with MTN for a Samsung Tablet from March 2013 and I ensured that every debit was honored during the contract term until it ended this year in March. My normal monthly fees was R350, but when the contract ended in April I was charged R180 and in May I was charged R694, which I took it up with MTN through emails on the 22 of May 2015 since they don't answer calls, to inquire about the overcharge when I never over-used the tablet, Guess what? I was only rep**** after a month on the 30 June and they said They can't help me since I did not pay the outstanding amount and the account has gone in arrears, how was the account not gonna go in arrears when they took a month to respond? I called them multiple times but the keep hanging on me. By September the money in arrears has reached R1000 and now they threaten to Blacklist me if I don't pay. So which means we should just pay what ever they charge us without inquiring? I am going to ICASA! I was never given a chance nor was my query attended so I wont pay the money that was charged without an explanation neither will I pay the money in arrears which they made to the contract incur while I was waiting for their respond.