Vodacom over-billing

I recently changed my bank account details, and as a result, my Vodacom debit order for my cellphone contract on Jan 25 bounced. I then called Vodacom on the 26th to update my banking details as well as change the debit order date from the 25th to the 31st, and was told that I had to go to Standard Bank (or a list of other banks) to settle my account, and that from Feb onwards the debit order would go off as usual.

I then went to Standard Bank on 30 Jan to settle the 855 owing, and received a confirmation sms from Vodacom that my payment had been received. On the 31st of Jan, not only did the debit order for Jan still go off, but it was for 1055, an even higher amount than my invoice of 855. I called the call centre today and have been on hold for 30mins. I've obviously instructed my bank to reverse the debit order, but I'd still like to know what exactly I was invoiced for, as I expect it to be 855 every month per my invoice, and not the 1055 they took from my account yesterday.