Vodacom unable to add my VAT number to a previous invoice

I have spoken to 5 different departments in an attempt to advise them of my Vat Registration. I have been told that they cannot add the Vat number cannot be added to invoices already issued - how is this possible when billing is done in advance? Of course I cannot speak directly to the billing department either! I was advised to speak to SARS. So SARS needs to allow me to claim VAT without my VAT number reflecting (which is law) because Vodacom's system does not allow for this? Unbelievable