MTN expect me to pay for an upgrade phone that does not work

Upgraded my contract at MTN Boksburg Rietfontein rd. The iphone 8 is not working and they expect me to now sort it out with iphone store itself. They refuse to cancel the upgrade from the store and the MTN customer complaints devision only refer you back to the store. I now do not have the upgraded phone any more yet I am expected to pay for it. Worst customer service ever.