SONY EXPERIA M5 - contract renewal

1st week -Could not receive inc. calls, sms's, when I phoned a client, I was speaking to a unknown person and so was my client, then the call switches back to my client. Calls ended during conversations, phone switched of and could not restart without charger. Went to cell C the 5th day after renewal of my contract. Requested the consultant to exchange my phone on which he rep**** he can only exchange defected phone's and that the problem must be with my sim card. He insisted it is a settings problem and said cell c will send me a sms for settings within 24hrs. Did not receive the sms, phoned Cell C, told them phone is still not working right and I did not receive the sms yet and we can not establish if the problem is with the phone or the card and that I would return the phone after the 7th day if the problem continues, due to the late sms. I went to cell c a few times before leaving for holidays, still they refused to exchange. After holiday, I went to cell c, they insisted to send it in for repairs, it came back, still not fixed, I again requested exchange, they refused, I left the phone there, they insist again to repair it. I want a new phone been paying for no service.