MTN Sold me a faulty iPhone on contract then failed to resolve the matter

I bought a mobile device at a mobile store on a contract basis. I was informed by the sales agent that I have 30 days to return the device if unsatisfied with it for whatever reason. A week into having the device I experienced glitches with the speaker, where the volume just suddenly decreases out of nowhere whilst I’m watching a video or in a call to a point I can’t hear. When pressing, the ‘up’ volume button will respond when I press it but only showing the volume display as being increased at that time but have no effect on the actual suddenly low volume. The day before I had called the call centre and an agent confirmed the 30 days to me. When I go to the store to return the device ( maybe like 27 days in or so, It was a busy time in my life) another sales agent attended to me who now said actually the device could be returned in 7 days of purchase …complete new news to me as I was told 30 days from the day of the purchase (bear in mind its my first time entering into contract for a mobile device). I obviously inform the store agent this, and that additionally a call centre agent confirmed this to me the day before, A complaint is then lodged with the store manager. I informed him I want the contract cancelled, he pleads that I let him book the phone for investigation then they will issue a new phone if they find it’s faulty. After much persuasion I give in and he says he will contact me to arrange the drop off of a curtesy phone and collection of my faulty one. Weeks pass no response. I escalate the matter to a higher manager, who also fails to assist after the same promise. I then lodge a complaint with ICASA. ICASA comes back with the same response as a ‘copy and paste’ response that’s been provided to them in response to my complaint and guess what? The response by the store is still to book the mobile device for that investigation…and that apparently it me now who was informed to book the device…note the “I” book it and not them (new news again). I responded with my dissatisfaction with the proposed resolution on the basis of this being the very reason I lodged a complaint with ICASA and I proposed that the contract is terminated immediately they collect their device. We are approaching a 6 months period since the phone was purchased and ICASA gone silent (last communication was 14 Aug) on the matter. I’m stuck with a faulty phone, that was bought under false information for a period of 24 months and it’s it fair. I need someone to advise how I can be assisted, this cannot be legal!