Increase in MTN monthly costs - off contract

Good afternoon. I have been a client of MTN's for many years. My contract expired in August 2017 and I chose not to upgrade my phone or renew the contract.

Since September 2017, my cell phone bill has increased fairly dramatically. My average bill on contract was R 500 per month. This included paying off the handset. It is currently averaging over R 700 a month. I have paid R 1100 and R 959 for monthly cell phone bills in the last 6 months, which is very unusual for me. These amounts do not include the top ups that I have paid via my Nedbank and Capitec accounts.

I emailed MTN on Thursday the 5th of April at 6:02 PM regarding this. I have still not received a response.

I would like an explanation as to why I am paying more? If a satisfactory explanation is not supp**** I see no point in remaining a client of MTN. There are other options.

Thank you.