Threat to suspend service without notifying

Last month I received my statement as normal, my debit order went off as normal on the 25 May. On the 2nd of June I received my statement with an amount of R521 in arrears. I phoned your call centre who told me to email my statement through. Which I did to [Email Removed] The lady told me the R521 was an amount oustanding for my upgrade of my contract wihich was not deducted at the time of my upgrade. Which I did not understand why it was only being deducted now instead of 3 months ago when I upgraded my contract. But I accepted it and agreed to have the amount come off with my next debit order on the 25th of June as the statement indicates. But then I get an sms today to pay the R521 in or else my service will be suspended within 48hrs. Now my question is how must I know I am in arrears if you dont tell me. All my previous statements does not indicate I owe you the money. If your debit order does not deduct the correct amounts. How am I suppose to know you did not do what you were suppose to and did not tell me. My statement on the 02 June is the first indication that I need to pay you R521 extra. I will not pay that amount in now, as I agreed you can take it off on the 25th