Unauthorized debit order

I upgraded my cell phone contract on 18th June 2018. i received my phone and a free phone which came with the cell phone package i selected, everything was good. On the 26th of June 2018, to my surprise i was debited with an extra R473.66 . as far as i understand i am only suppose to pay my new amount in July. I contacted vodacom on the 26th to enquire abouut the extra amount and no one can explain what the extra amount is for. I was then told that they will escalate and that i will be contacted within 24 to 48Hrs, however no one has called. I need vodacom to refund me and i need assurance that i will not be debited extra again!