MTN Mimosa Mall Service Complaint

On 04/06 I went to MTN Mimosa Mall branch to upgrade my contract. We chose a Samsung A31. We returned to MTN on 08/06 as the phone had developed software problems: System UI interface, camera and flash light not working. As this was within the 7 day period we were given a new A31. We returned this second phone to MTN on the 14/06 as it had developed the same problems as the first one. Again we were within the 7 days and were given a new A31. 0n 20/06 at 7pm this 3rd phone sent a "system UI interface" error. Being the 7th day we sent a WhatsApp to Chad (the MTN employee assisting us) who recommended that we return to MTN the following day being Sunday the 8th day. When we went to MTN they refused to replace the phone and advised they now have to cancel the entire contract as we are 1 day late. They did sent emails to all concerned to advise we had been in the shop. On Monday we went through the same story and were advised by Suzette the shop manager that we will just have to wait until their retensions department decides to read the mail sent by MTN Mimosa to cancel the contract. This should take between 24 and 48 hours. We were also not allowed to hand the faulty phone in which is strange as we were previously told that we had to hand the phone in as soon as we notified them of the fault. It is now Wed 23/06 and we were advised by Chad today that they have received no response from retensions. I have been a customer of MTN for 21 years and have 3 contracts with MTN. Besides this the A31 series obviously has a software problem which needs to be rectified. This has nothing to do with me. Neither do the internal systems of MTN have anything to do with me. MTN are a service provider who have not provided any service here. I am 20 days into a new contract with no service. I do not agree with this and also find it odd that this "retensions" department cannot be phoned, only mails. Disappointed and frustrated beyond.