I am being forced to pay for a faulty device i returned to MTN after 3 days of purchase and this happened in 2019!!

Got a J7 contract via Mondo. It was delivered on Friday 27th Jun 2019 Account No. A***. That Sunday the 30th the fone was faulty wrote an email to "***********" telling them I want a new fone from the box, got a response from Phindile email "customercare@buymondo' to return the device to the nearest MTN which I did at Canal Walk Century City and they gave me this reference WO*** when I returned the device. Tried calling when I was in the shop and the guys promised to call and let me know. Now I am being harassed for a device that I returned and I was handed over? If yo wanted me to keep paying why did you not return the faulty device? Where is it? Why should I pay for something that I do not have? I need my name cleared ASAP, please locate your device, if you feel I should pay please give me a new device and I will gladly pay what you feel is due to your company.