Bad Service & manipulating me into a contract I didnt want

I used to be on a MTN My Choice contract, where I got 350 minutes, 500 sms's and 500MB Data for R371,83. Then MTN phoned me in September 2018 and said they can give me exactly the same for R249, so I agreed (who wouldn't). Only to find out 2 months later that they changed my contract to a Choice Flexi R350 which only gives me R350 Airtime, by the middle of the month everything was depleted.

Now I am trying to get it changed back. 1st I was told to go into a store, according to them I need to pay for the total amount left of the contract if I want to cancel it (R249 x 24) or my other option is to change the contract but then I have to pay an additional R100 on top of the monthly fee every single month.

Now my question is: Why is it so easy to change my contract over the phone to something I did not even want? But now that I actually want to change it, it is impossible.

I have also been redirected to 3 different number to call after I phoned 135.