Debit

I am struggling with Telkom and over charging for no reason. I took out a data plan with a small pocket router for R230 .. which is R200 for the data 50 gigs and R30 for the pocket router each month for 24 months so that the router can be paid off.. I then received a call to say they have an upgrade of 120 gigs for R60 more to what what I was already paying and I agreed to it because I know I will have atleast that Amount of money left in my bank account, but when they debited me at the end of the month they debited me R850 which I couldn't understand how they got to it.. they said they have made my router obligations once off and added all the amounts I was supposed to be paying off for 24 months in one bill, which is something I never agreed to, the debit order bounced because I did not have that amount of money in my account. They then disabled my data .. and I ddnt use it for months until the again debited me 2000 Rands that was successful, which I still do not understand where they came to that amount .. then still had a nerve to swmd an invoice saying I still owe an amount of 2900