Wrongfully billed and MTN is not investigating they only want me to pay the bill

I have a top up contact for all my phone nr with MTN. On the 8.10.2017 I got an SMS from them stating that I am over my credit limit, and that are going to suspended my contracts. So when I phoned to inquire what the problem is they could not really assist but reactivate me service. I receive me bill on the Tuesday that on my one nr there was 50MB been bought for R 2850 rand. According to the them we have used the self service but when you are on a top up you can not use the self service as there is not credit line. I have launch my complaint with MTn on the 10.10.2017 and up and till today no feedback, according to what I understand MTN accounts department did not even look at the issue as yet and also the person assisting me now is trying to see if she can get proof of how this has happen but to no avail. The thing is they just think that you will pay this and I am sorry I am not paying for this. Please note MTN is not here to help you resolve your issues they just want you to pay. The funny thing is 1 GB is costing R 160 how do they get to that total. You can not call 808 as you never get through to them it keeps cutting out.