********** action on my contract ref

I have a contract with MTN that I signed for on the 15/06/2018 for an Apple phone 7 32GB under MTN Made for me S @349. I added data package which made it to be R549 of which it ran smooth for months.

Last month all of a sudden my Debit was R 2032,99 and I logged a complaint and today I was told that the extra charges was a Pro-rata charges because my contract was on special for 24 month so now since it has expired they are charging standard fee. The reference number I was given for my complaint is 1-**********

1. When I signed for this contract it was never stipulated to me that after 24 month it will change.

2. How do they charge a Prorata that big whilst my contract is in its 24th month and I never even cancelled or did anything to it?

3. I've checked the original contract copies and there in no where were it has that stipulation.

Now they have suspend my number and when I call fo enquire I just get new cases that get closed without being solved. I have about 5 reference numbers with 1 case that has no solution.