Complaint - INCOMPETENCE in updating receiving tax invoice

I have phoned in for 3 months in a row after upgrading my account complaining that I am not receiving my tax invoice on my main line, and yet again, after assuring my last month that the billing method has been corrected I have yet to receive my tax invoice for the number to which all other accounts are linked. I cannot believe that you cannot get this right MTN!! WORST SERVICE EVER