BECAREFUL BUYING ONLINE AT MTN.

Good day. I have a problem regarding mtn. They call me every day requesting payment regarding a cellphone device I don't have. So, what happened was: I app**** for an iPhone, they sent me a Samsung. I returned the phone. I even took it back to one of their stores. I requested a cancellation letter MULTIPLE times. their call center agents put you on hold or they lie and say they going to send it. until today, no cancellation letter. but they can call me regarding payment. I WILL NOT PAY MTN ANYTHING!!!!!!