No Device but I owe MTN

So, I took out a contract in 2017 online, my package was delivered. 2 days later I returned the package as it was not a dual SIM as it was supposed to be. I returned it to a store in Canal Walk Cape town where I was assisted by Mike Moyo who at first refused to take the package as it was not wrapped the way it was when I received it. We argued about this until I requested to speak to a manager which also refused. Bare in mind this box they want is the big old box the package was wrapped from. The original cellphone box was there, with the accessories as well. I then refused to leave the store and suggested that they call MTN warehouse in JHB which they did. JHB told them they can accept the device as is. They took the device then and sent an email to JHB, and they rep**** to say they can take the device and send it to them. I asked them to cc me in the email which they did. Eversince that day I never heard anything from MTN until one day when I received a call to tell me I owe them R1,772. for the device I took. I have all the proof of numerous email I have sent to prove that I do NOT have a contract with them. They never cancelled my contract nor given me another device to substitute. I am sitting here with an amount that I must pay for their negligence. I want someone to answer to this!