Not sending invoice

I have been receiving my monthly invoices via email from Mtn every month until this month after I have upgraded my contract. My billing date is the 14th of every month. I have not received this month's invoice and I need it to claim payment from my employer. I have called the Call Centre 4 times already and everytime I was told that they were sending the invoice as we spoke. I just need someone to investigate why am I not getting the emails. I really need the invoice