Dispute on Account

I cancelled my contract with MTN long time ago a after realising that the contract was not what has been agreed upon.I then asked the agent to even articulate on the contract as it wasn't clear on the contract. Subsequent to that, I got the account statement and to my surprise i was over charged more than what was agreed upon. I then lodged a dispute which has been going on for the past 8 month,i have been following up with MTN Montana but i have never received any positive response, the only response I got was that the matter is still under investigation until surprisingly i started getting calls from creditors that i owe 7 months interests. I fail to understand the reason why will i accumulate interests on something under investigation. I want to believe that MTN is using any opportunity to rip disadvantaged individuals off, Much as there is a dispute on the account, how can I be liable for any intersets.