Annoying calls about up-to-date Account

I'm getting about 3 calls a day about an outstanding amount on CellC account. Each time I phone this number I'm getting told that there is an X-Ammount outstanding, but more Proof of payment have been sent, and already acknowledged by *** more than 2 weeks ago. Is it really this difficult to update your billing system so that all departments can see the payment made 14 days ago ?