Shock Bill - QUADRUPLE INCREASE

My contract ended and I was called by MTN to offer me an upgrade. I explained to the lady calling that I want a sim only deal, she should look at my usage and current package and then recommend a suitable one. She said she would call me back. A month passes, same situation plays out with a different consultant. Nada! Then I wake up to an invoice whereby my bill has gone from R419 to R1785. After calling 135 for almost an hour, transferred 4 times and still no solution, I don't know what to do. Please MTN, contact me and sort out this crazy invoice, due to your staff not following through. I have been with you for over 20 years, this is not AYOBA