Charged for Device that dont have any longer

After getting faulty iphone 12 on business contract was told by staff at Watercrest mall mtn to wait to return it in the meantime battling to make and receive call which went on for about 5 months then when finally got them to return the device, They cancelled the contract due to no available replacement units but wont do any refund for months that paid for device that they have now taken back without replacing

Shocking !!!!!