unrepaired handset

I have upgraded my contract 2 months back and since then I have been having technical issues and problems with the handset. I have sent it in for repairs which is under warranty and also has a 6 month exchange guarantee but the handset even been repaired still has the same issues and has been sent back after 2 weeks and now I'm experiencing more issues. This is poor service and I am highly disappointed with the service I received from the mtn network and to top that I am waiting 30 minutes for the repair centre to answer my call this is poor and pathetic only to be cut off. The handset is still brand new with the original screen protector plastic it came with. This is uncalled for and the worst service from mtn and after my contracts equal out to +- R900 per month.