Cell C Billing problems

We have a contract with Cell C. I've been contacting Cell C for the last 2 months to sort the billing of my account. I struggle to get Invoices from Cell C after rectifying my email on numerous occasions. Cell C billed me twice for November 2018 but cant explain why I need to make 2 payments for the same month.

We upgraded on 29 September 2018. My new contract started on 1 November 2018. In November I was billed the amount for the old contract even though I receive only the data & call time from the new contract. I've contacted Cell C to sort the bill but they advice that the bill is correct & need to be paid. December I received a bill for December and Pro-rata for November .

I again contacted the branch(3 times) & the call center. They can't tell me why I need to pay Twice for November only that I received the data for November & I need to pay AGAIN for November.

I still do not understand why they are fast to push for an upgrade but they are not able to do the admin & billing correctly