Won't pay back the money

I have had two cell phone contracts with MTN for a couple of years. In 2016 I ended my first contract and in 2017 I ended my other contract. Both contracts worked on debit order payments, which means MTN deducts the money from my bank account. When I cancelled both my contracts, money was still deducted 2 - 3 months after cancellation, and then it finished. Now all of a sudden I get notification that I owe around R440 on my one contract. Now my question is, how on earth do I still owe you money, after you deducted it via debit order and the debit order ended when the contract ended? And why now, after almost a year, do I all of a sudden get a notification of money owed? This is unacceptable, since I did everything that I was supposed to do, and I did not even cancel the debit order, it ran its course. I will not pay the monies owed, as this is a fault on your side, not mine.