Unhappy with Telkom Mobile

My WiFi contact expired 2 months, I was on Telkom 40/40Gig. Before it expired, I phoned them in order to cancel and some salesperson said that I must cancel it exactly 30 days before expiry. When 30 days before expiry came, I cancelled the contract on their online platform. I even received an SMS confirming the cancellation with the following ref. number: ***9. Two months later they are still taking money from my bank account. They are no longer giving me 40/40 Gigs as per initial contract, they are now giving me 20/20 Gigs. That means I am paying R352 for 20/20. Gigs. I believe that this is unfair, I have cancelled the contract by giving them 30 days notice and they are still taking my money after 2 months from cancellation. I cannot go through on their call centre as it is always busy.