Null belongs to Telkom, so they are perfectly right to charge for it. You probably called their call centre and asked to speak to the supervisor. There isn't one, so it's a null.
Of course it may be an "error". It's amazing hoe much money can be made from 100,000,000 errors ...
When you demand the refund, tell them to add interest to it jus as they will do if you don't pay it.