Unauthorised purchases on my accout

I've been paying around R600 p/m for a contract with three devices and in October it suddenly jumped to R2250. On inquiry i was told i purchased it using USSD, not really understanding what it was. The salesperson i spoke to at the Westgate Telkom shop forwarded a query to the relevant department and the following day the response was that i bought data and airtime using my phone. I explained that i did not do the purchase because i am not a big using anyway and pointed to my previous monthly usage which would indicate how much i pay on what i use. This was not taken and into account and after much insistence another query was opened. A month later another bill of R7000 is sent and i am now owing R9000. If the first query was promptly attended the second one wouldn't have come about. The said part is that i am not even given the benefit of the doubt where Telkom says they will investigate and revert back to me at the earliest. Instead, two months later there's still no feedback from anyone and they've decided to rather suspend my devices. I have resorted to cancelling the debit order and will only pay once this matter is resolved. All i need is to find out who and how data and airtime was purchased on my account.