Incorrect Amounts charged to account

During September 2014, I took out a cellphone contract with MTN. Since then I have paid every month and my statements reflected the payments. I paid the amounts shown as due on my statements. Since Mar/Apr 2015, however, my services have been disconnected 3 times. I phoned in to find out what was going on and was sent an open item statement. The opening balance on the statement shows an amount of R1825.28 on date of contract inception, which MTN cannot explain to me. The case has been escalated twice already, but I have had no feedback. Once again my service is suspended and I cannot reach anyone via email or the call center. Now I am being threatened with legal action, and yet, MTN has not been able to tell me why there is an amount of R1825.28 on my opening balance, even though my statements from that time had not once reflected this amount. I refuse to make any further payments, until MTN can undoubtedly prove that the opening balance is indeed my responsibility, which I cannot understand, as the contract I have is for a fixed monthly amount and I could not have made any calls on the day the contract started.