Mtn contract cancelation

I took out a mtn contract in 2023, as per telephone, was promised a Honor 9 duel sim, received wrong phone, so I decided to cancel the contract. I've returned the package to Mtn Store in Century City Cape Town. Package was booked in, received reference number and number to call to confirm receiving of package. I struggle with Mtn since 2023 to sort this out. They handed me over to debt collection as I owe them more than R8000 for non payment over the past months. How can I solve this, as the phone package was booked in within the cancelation time frame. Please help