Cell C don't know how to allocate payments

This companies accounts department has absolutely no idea what they are doing. Why am I saying this. Well, for the past 6 six months I keep getting a phone call from CellC to say that my account is in arrears (which is not). I mail proof of payments and their spam filter removes the payment attachments and I'm then told that the emails don't have proof of payments. Every month I pay and every month they say I don't.