Incorrect billing

My contract ended in February, I was still using your service on a month to month basis. I ported out of your network on the 3rd of July - after much hassle from your side. You changed my account from post paid to pre paid so that I can port out of Cell C then. Now I see that you have debited me R399 today. I have been out of Cell C for a month and am still being billed? If you want to charge me for the 1st to the 4th of July - fine, but you should charge me pro rata, as is the norm in the industry and not charge me a full month's contract - when I have been changed to pre paid - and ended my service with you a month ago. Please refund my money and stop debiting my account. Although I am no longer a Cell C customer, I still expect good service and trust that this will be sorted out asap in good faith - as I might still be a Cell C client in the future. If I get poor service from you I won't return to Cell C and will spread the word to others to do the same. Thanks for assisting - please sort out the problem, refund me and stop billing me.