Billing error

I took out a wifi contract on 30 May 2018 with a fixed monthly subscription and data. No default limits selected and access to the internet is automatically denied if data rans out. I ran out of data in July, could not log onto the internet and billing was fine. In September 2018 I was billed R500 extra for data usage. I disputed ( ********** 3 & ********** 1) it but Telkom is not responding. I called and emailed but to no avail. It's clear that Telkom can get away with this malicious conduct.