No solution to payment allocation

I made payment on the 25th of July to mtn using my Nedbank account, normally for big companies like mtn the bank would have bank approved beneficiary to make it easier for clients to make payment. I then received an sms to say they did not receive payment and they blocked my phone, I then called mtn and they told me that I need to send the proof of payment and they will allocate the payment, I called so many times no one was able to assist, I eventually sent an email to their complaint department, they told me to still send the proof payment and it will be sorted, I eventually thought it's corrected until today on the 25th of August, they double deducted, when I called they said they still did not get the payment for July I find it hard to believe that a big company like mtn is unable to assist and double check their books / statement for the payment and allocate it, in June I paid them using an eft and they were able to make payment, what is the difference this time. I honestly regret the day I changed from cell C to mtn.