MTN Reaping people off with ********* behaviour

I had 24 month cellphone contract with MTN and the contract ended last year.i was getting 20GB at R500 and in addition MTN was giving a free 20GB. If you don't call and cancell the contract when the term lapses they automatically put you on month to month basis ...while previously as far as I know .The contract just get cancelled automatically.

So now on a month to month they the charged me Four times what I was paying before.

And in January thats when I became aware and I canceled the contract after having paid R2 173 instead of my Normal R500.Which I ended up paying because they even disconnected my line

To my surprise in February R2 643 was deducted from my account and I called the consultant and try ingTheir reason is when I canceled on the18th they have already issued out an invoice on 12th of January

So now I must pay R2 643 on the basis that the invoice was issued on the 12th while the contract was canceled on the 18th ...I found it very uncold off and *********....Just want to reap people off.