Double charge

I upgraded my contract at the end of June 2020 but the upgrade took effect on 04 September 2020 as it should have. At the end of September I was charged the full price of the old contract but my benefits of that contract was terminated on 04 September. At the end of October I was charged pro-rata for the new contract that took effect 04 September. When I contacted the call center and informed them that I was being over charged I was told that I am not. When I insisted to speak to a manager I was told one is not available and that a manager will call me in 3 to 4 working days. To date no one has returned my call but the double charge was deducted from my bank account.