Poor customer service and no resolution in resolving an incorrect statement on my account

I've had the absolute WORST experience with CellC over the past couple of weeks. I have been with the service provider for over a decade but I am considering never using them in the future again. I cancelled a contract I had with them, effective 2 July, yet they still sent me a statement to pay the amount in full for the month of July for data I have not received since the data contract was cancelled. I should only be paying a pro-rata rate for 1 and 2 July. All emails to their customer care department have gone unanswered and calls to their customer care line is no use either. They keep transferring me from one department to the next, where I have to keep explaining the issue, and when I eventually get put through to Cancellations, no one is available to field my call. This is absolutely terrible customer service and I am at a risk of having my credit record tarnished since I will not be making this payment but they refuse to amend it to the pro-rata rate I should be paying. Why am I expected to pay for something in full when the contract was cancelled and why is no one able to assist me in resolving this?