double charges

Very frustrated with the service from mtn, as a previous nashua mobile customer my contract got transfered to mtn, I decided to upgrade my contract on the 20th month, I asked if I am going to pay for the balance of the 4months on the old contract I was told no. When I receive my bill at the end of the month I am being charged hardware fees for my old contract and my new subscription. I am not willing to pay for the hardware fees and new subscription. Its the 1st time i heared of this, which I think is ****. I set a limit on my contract bt I have been over charged for data. When I called the call centre they said they will get bak to me in 7days. No1 had gotten bak to me. I had to call for feed bak. When I askd for feedback they said that issue has been resolved, how can it be resolved if the client does not no whats going on.i want this sorted out now!!!