Billed for items not agreed upon

I upgraded my Telkom mobile contract in October 2017. The new mobile phone was delivered on 25 October 2017 and contract was effected the same day.

I have received the invoice and to my surprise was invoiced the the full device fee for the month of October 2017. I was informed of a pro rata amount when I agreed to the contract but not the full amount. I am aggrieved and not prepared to pay the full device fee for October 2017 as this was never part of the agreement with their sales consultant.