Carryover/obligation fee

I upgraded to a device last year in April 2023, the package was a Flexon 6, my installment worked out to R699 for 36 months. When I received my statement I found out that I will be paying an amount of R1018, an extra R402 carryover/obligation fee. When I asked Telkom they said it is a carry over fee from the previous device and it will be deducted for a few months since I upgraded months earlier. Now 9 months later when I enquire I am being told that the device obligation fee will carry on until this year October but nobody can explain to me what exactly is it for and why do I have to pay for so long because my total package of the device was only R699 until the expiry of my contract. I still need answers while I am being ********** every month.