MTN CONTRACT

i have bought an iPhone through mtn last year in September and i received wrong device that i have not opt for. I called mtn and i was advised to take the router back to the nearest shop which i did and i was told that as soon as they received it from their warehouse, they will cancel the contract. My complain is that I keep on getting emails that i must pay they actually billing me for a device that I don't have. kindly help on how do i clear my name because now i am threatened that they will blacklist me. I HAVE PROOF THAT I HAVE COMMUNICATED WITH THEM