Please get your financial systems in order its not working

For the past 6 year and 8 years being with Altech auto page and MTN respectively I've been going on well and MTN has received their payments monthly without any problems. But this month my payment goes from R755 to R1522.66 when I called the call Centre I was told it was due to a payment that wasn't directly deducted in April 2016 which is almost 2 years ago , this is seriously a joke as this amount never showed up on any invoices under the 30 ,60,90,120 etc tab. How is it 2 years after I've been with MTN this comes up and how do you manage your systems no wonder people are always complaining about your services cause of these mistakes. It's not like I budgeted for this total and I've spoken to a lady at your call centre on Saturday the 28.07.2018 who advised me that it will only be R755 that will be deducted so when I called today how is it something different . My advise please get your house in order cause recommending MTN is just a no go for me.