struggling to cancel a paid off contract.

This is the most unprofessional company that I've ever dealt with. I had a cellphone contract with them which was paid off a year ago. I don't know how it happened but one of their agents signed up a sim card contract for me which I did not approve. the are constantly deduction money from my account every month and the amount keeps on increasing (R1600 this month). I have never even used this sim card (using MTN). when you go to the branch to cancel they give you a number to call with zero assistance. calling this number is also a waste of time because no ever picks up. this is affecting my credit score.

I am never getting any contract with Telkom ever again and I'll never advice anyone to join them