Shocking Call Centre , Poor Billing and No Answers from CellC

I paid my settlement amount on my phone in July 2017. An amount was then deducted off my account at the end of July which appeared to be the normal monthly installment minus the phone (which was now paid off). I then received an invoice from CellC in August 2017 which made no sense. I tried calling CellC to get clarity on what this amount was considering I was under the impression that my account was settled. Maybe there was an amount that I owed. However, going through the CellC call centre is next to impossible. They no longer have me as a client, so they are unable to verify who I am or what the situation on the account is. Yet, they are able take my money.

I need to know if this amount that was taken off my account is legit. If so, then so be it. However, navigating the call centre (084145) is next to impossible if you are an ex-customer. And the mere fact that they are unable to pull up my information is ridiculous.