Overcharged by upgrading on my date given by MTN

I checked on the my MTN app and on the MTN website to see if I am eligible for a upgrade as my 24 month contract is nearing a close. I got my upgrade date and used the MTN online website to do my upgrade after MTN confirmed I can now upgrade and someone from the Call Centre also phoned me to ask if I want to upgrade, to which I rep**** I am upgrading online. Nowhere during the process online did it mention that I need to pay a penalty or early termination fee as this was just a normal upgrade like I have done with all my years before at MTN. Even on the website and invoice I received with phone it showed R0 due. To my surprise I had a debit order on my account go off for over R3200 and my monthly payment is only R899. I tried to phone the call centre 10 times or more and got no answer. I went in store to get help and they said they can help either. Is this the way MTN is treating their customers? I will not leave this matter like this. I want answers and this corrected or I will take this matter to the highest level.