Overcharging me for fees from 2016!

I changed my payment method from debit order to self-pay, since MTN cannot be trusted to debit my account correctly. In October I was told that I had an outstanding amount from 2016 that I needed pay to avoid my lines from being suspended. Needless to say this was from a previous contract that I had in 2016, which at the end of the day is no longer my problem. I ended up paying the R499 anyway along with my monthly payment. Now I am getting an sms from MTN saying that I have "agreed to pay R937 for 2 weeks as of 27/11/2018 for an overdue amount on my account".

1. My monthly payment is R533 as I am on a fixed contract. I did NOT agree to pay you more than what my monthly fee is.

2. I already paid the overdue amount from 2016 that quite frankly should have been written off by now - as I am not even sure that there really was an outstanding amount. MTN is sucking me dry when I have fixed contracts and pay my monthly fee of R533 religiously every month on the 28th! You need to get your house in order. I have never dealt with such incompetence. It seems to be your billing department that clearly has no clue how to count. To go into the sore is even more useless than trying to sort this out with the call center.

I need someone to sort this out asap as my lines have been cut twice a month now for 3 months in a row when I am a paying client!! I never miss a payment!