Incorrect billing

Went for an upgrade in June where they pulled a statement and told me that end of June the last debit will go off for the device.

Received July statement and again they're charging me for the device that is already paid off according to their system, when I phoned to query this they told me I had another month left and that I have no choice but to pay this .

I told the agent that the shop where I upgraded pulled a statement and it showed 1 more debit left for the device in June , her response was I don't know what system they use as its wrong ??? I take it Vodacom doesn't use the same system across the board??