Bank account not debited

I January 2016 I called to change my capped line to an uncapped line, this was done. I was told that the extra amount would be around R150.00 more than what I was currently paying, so this would mean that I would be paying around R620.00 a month. My account was debited in February. I then went into the Telkom Canal Walk branch in Feb to change my debit order details which I on a piece of paper, but my new account was not debited at the beginning of March. I am now sitting with a bill of R1970.30 which i assume would be for the March debit that wasn't done and for the one that is about to go off. My first question is how is the total that much if the monthly debit should only be about R620.00 and my second question is, have my debit order details been updated? Please contact me urgently as my internet has been suspended and I also need an explanation before any debit orders do go off, if in fact these details have been changed.