MTN poor service

Im frustrated with the service which I’m receiving at Mtn . i bought an i phone 7 plus in August 2019. within two month the phone was no longer working. I returned the phone september 2019. till today I’m receiving different stories when i call to find out about the device. they told me that they can’t track and trace the phone when they booked it in for evaluation in midrand . I’m frustrated because every month I’m expected to pay the amount of R450 without the device. i really am frustrated