Ruining my credit record!

I got an Iphone7 from MTN through Rewardsco last year on the 10th of December. The phone was faulty and i returned it same week (14th of December) to Rosebank MTN so basically i did not use it much and as far as i know, you can return any goods within the same week you received them. I kept communicating with MTN and Rewardsco. MTN has now handed me over to their lawyers for a phone that i returned last year and not used, they say i owe 15000 WHAAAAAT?. How does that work? I have been phoning these two providers every month since this year started to check the results but no one is assisting me. Guys please be careful of these two companies and I beg, now they are ruining my credit record for nothing! I have been communicating with Poorvasha Naidoo through emails from rewardsco regarding this cancellation till today I am not getting any help. When i phone, she is unable to come to the phone. MTN please tell me who do I speak to resolve this matter because I am really getting tired on this, no wants to take any responsibility for this. I just need my name cleared!