MTN give me an account of R2473.51

I have an account with MTN for the past 10 years now. Last year my MTN cell phone contract was transferred from Nashua to MTN when Nashua did not continue with the contracts. I visited MTN Brooklyn square to combine my 2 MTN account and other problems that I encountered on my MTN cell phone contract since they took it over from Nashua mobile. I was under the impression that the 2 contracts was indeed combined but the other problem still existed and I it took MTN 4 months to sort this out. Then in May 2015 I received and account from MTN for my existing account that was never paid as it was not combined with the other contract. I paid the outstanding amounts on the account and received a statement every month of what my account is. Then in July 2015 I suddenly received a call from a debt collector telling me that I owe MTN R2473.5. I queried this as the account nr that they supp**** to me was not the same as the one I have with MTN. I supp**** them with the account that I received from MTN but they are harassing me everyday with the account and I want proof from MTN that I do owe them the amount they are saying as according to there account send to me I am up to date with my account