MTN useless and will not assist me *** this is my friends number whom I live with

Bought a phone in January R3500 from that day the phone have never been in use it’s a multiple user phone which I stipulated I did not want every week they give me a new SIM card the phone is NOT useable MTM will not replace the phone it has been to head office and they will not replace the phone. Please someone help me I am a pensioner and need the use of this phone which . It was supposed to be a new phone and it’s an old phone I am desperate please assist me I am desperate. Every Monday they change the sim so I do not have a number what can I do. MTN will not assist me