Double billing for contract: case ***

I have upgraded my Cell C contract to a Pinnacle Unlimited R 999.00 in February 2018. At first the Vincent Park Cell C shop gave me the wrong phone, which I returned and then received the A8. The upgrade was reversed and a new one signed for the Samsung A8. I paid off the 3 month that was still on my old contract to enable me to start on 5 March with the new contract. Since 16 March until 16 April I have been billed R2 603.91 for this contract. I have phoned numerous times, have a query ref no. ********** 106 (13 April), e-mailed Cell C, with no response. Every time I make enquiries, I am told that the "back-office" is still listening to the voice recordings. It is impossible to get hold of anybody who is in a position to solve this matter. I am very angry and frustrated with Cell C and regret taking the new upgrade.