Refused to cancel the returned phone that I did not even unbox

So I bought two phone on separate contracts and it’s vry easy but when I recieved my phones a day befor delivery I lost a client so I knew I wil nt afford the two phones again,I returned one after 5day MTN stores like two stores returned me that I should go to the bigger stores later I tried calling mondo it was weekend their phones are off Saturday and Sunday Monday I went back to store in Santon this was 11 day,the guy book in the phone for collection they refused to cancel contact because I returned the phone after 10 days I told them I hv been sent back and forth and weekend your phones are off and am also working I cnt be able to just leave work everyday to ,MTN got involved the phone was cancelled I started to get invoice for one phone ,only after two months of recieving confirmation for cancelled contract ,and paying for my other phone I recieved message tht I owe R6000 for cancellation fee ,their consultant they will promos to call you back they cnt ,sometimes the hang up the phone on you , I dnt want to be blscklisted please cancel that amount because I cnt pay it Mondo