Incorrect billing

I have a SmartBroadband Wireless 30GB Deal with Telkom at R549 per month. Since May 2017, Telkom has been charging me for additional "add on data bundles" despite my account being a standard contract that does not allow for out-of-data bundle purchases. And the amount that I am invoiced varies every month. 01 May 2017 I was invoiced R2672,33 and when I queried the account a case was opened with case number ********** 2. On 01 September 2017 I was invoiced R975,53 and on 22 September 2017 case # ********** 2 was opened. On 01 October 2017 I was invoiced R856,50 and on 04 October 2017 case # ********** 2 was opened. On 01 February 2018 I was invoiced R876,35 and on 08 February 2018 case # ********** 6 was opened. On 01 March 2018 was invoiced R660,39 and on 02 March 2018 case # ********** 2. On 13 march 2018 case # ********** 7 was opened again. According to the various call centre agents, they all concur that they are unable to see who/what keeps activating the “add on services” and suspect it is happening automatically by the billing system. They then log the call with the “back office” and “technical team” and when the matter is resolved I am promised it will not happen again. Yet after every promise it just happens again. And every month these inconsistently inflated amounts are debited from my bank account. This is very frustrating as it requires me to call every month and wait on average 35 minutes for a case to be opened, all at an additional expense as I have to call from my mobile phone that is with a different mobile service provider. Just today alone I used nearly R200 airtime just waiting for the call centre agent to review my invoice and log the case. I will be cancelling this contract as soon as it reaches the end of its term and will never utilise Telkom's services ever again. I have been a loyal Vodacom customer for 15 years and decided to give Telkom a chance. Never ever again Telkom!!