New phone in for repairs 4 times in 6 months

We bought (cash) new phone in April 2015 at CNA Uitenhage. First repair was booked in 05/06/2015 and received back 09/06/2015. Second repair was booked in 15/06/2015 and received back 22/06/2015. Third repair was booked in 18/08/2015 and received back 04/09/2015. Fourth repair was booked in 16/10/2015 and now waiting for it to come back. This phone is in the repair shop for longer as that we have had opportunity to use it. Every time it is the same problem. I was told by the agent that the phone will only be replaced after the problem persist for 3 times. Phone was booked now again for the 4th time (job no ***4) and I was told it was not going to be replaced...will only be repaired. If they couldn't repair it during the 3 times it was sent in for repairs why can't they replace it. I have a phone....but I don't have a phone....Who do we contact to insist on a new replacement phone?? Clearly this is a dudd phone.... why can't I be compensated for this phone??? I have demanded a new phone on 2 occasions, but my requests were ignored. It is unacceptable to pay for a phone that is not working!!!