Mtn cellphone contract

I took iphone 7 at mtn wonderpark mall last year September 2019,the phone was overheating within 3 days I took it back then they gave me another 1 after 3 weeks then it did the same story within a day I took it back after 4 days they gave me another 3rd phone after a month then I told them to cancel the contract since well all the phones they give me have the same problem but they didn't because they kept on debit the money each and every month last week I went to the store they told me that I must pay almost r3000 for penalty and they won't repay me the money they kept on debiting.They say I must pay it because I'm the one cancelling and I rejected the 4th phone maybe it was their last patch that had a problem...so why should I pay a phone I hardly used evn a week? All of them? Worse this is an inconvenience because I didn't have a phone at all that time that is the reason I wanted to cancel the contract because it was useless Please help so that I don't get to pay the penalty and they must refund me the previous money I'm only willing to pay the sim card minutes and data I've used.