MTN claims I did not pay up my contract.

In 09/2011 I app**** for 2 lines (Top-Up contract) at MTN Campus Square which were to be paid in 24 months.The agreement was that they would debit my account on the 25th of every month which they did. Shortly I got promoted to a permanent position so my pay date and account details changed, I called them on 083808 to inform them of the changes and gave an order that they should now debit my account on the 20th which they never did and for that reason I missed four payments and my account was in arrears. Soon after that I received a call from someone saying they calling on behalf of MTN regarding my account and they wanted to let me know that my account is with the legal department then I told them i'm willing to pay the account in full so they gave me the settlement amount and I started paying each month until the account was paid in full. I then called Mtn legal to request a PAID UP LETTER and it was sent to me on 18-12-2013 11:35 AM and stated clearly that everything was paid up and NO FURTHER PAYMENTS are required, to my surprise when I went to apply for a new contract a few weeks ago it was declined saying that I still owe MTN R214 and that's nonsense.I want this rectified.