Cancelled 2022 contract affecting 2024 credit score

I ordered a cellphone from MTN online back in 2022. I requested for it to be returned a few days later as it was not what I expected, which was in my right as I had the 7-day period to do so. A few weeks back, IN 2024, I get an email from a collections firm stating that I owe over R10 000 for a device that I do not have! This has severely impacted my credit rating, and every email I've sent has been a dead end. UN-PRO-FESS-IONAL!