MTN contract Issues

Poor service by far I have upgraded my contact in September on the 25 of November the debit order didn’t go off first time I called last week Friday I was told that they had system issues and most clients have been calling about this issue and no communication was sent, i was told that im due to pay for the old phone and the new phone no explanation call eventually cut coz they kept on transferring me. Today I called again to understand why I should pay R1800 the lady was very rude and told me that the answer is on the statement she refused to investigate why i was being bul**** on the old contract.please investigate and provide me feedback regarding the contract that ended in October why im billed for it my understanding is that when I did the update the new contract will only kick in once the old contract expires or end.