Mtn contract phone

When I upgraded 2yrs ago a load of subscriptions were unknowingly added to my account. After many months of having my phone cut every month and me having to phone MTN this was sorted or so I thought. It is now time to upgrade and I am unable to as there is an amount of R4500+ that I have been handed over for. Why? I am told I signed a contract for R600+ and have only been paying R370.00. I have a copy of the contract on hand where I signed for R369. I have been trying to sort this out for two weeks now to no avail.