Insane Contract period

How does this work?: @MTNSouthAfrica sells a customer a 24 Months contract, for some reason the customer will have to make 25payments (Installments) considering the fact that the customer never missed a payment and always been up to date with his/her debit order installments. Two of the MTN consultants say that the first (pro-rata payment) is not counted as your first payment. Can somebody please knock some sense with regards to this, maybe ndim oslow. I took a contract with @MTNSouthAfrica on the 07th Dec 2018 and the first installment (pro-rata) went off on the 27th Dec 2018. For some reason when I cancel my contract because it has ended, I'm being told that the last installment will be in December 2020.