MTN TAKES MY MONEY BUT CANT PROVIDE THE SERVICE

I recently upgraded my MTN contract on around the 16/12/2014. I opted for a Sony Z3 for R499 per month. The phone was faulty and was returned to MTN on the 09/01/2015 for a warranty assessment and was advised that I would get a response within 2 weeks. I did not get a response on the progress and had to call the warranty assessment centre myself only to find out that I was eligible for a new phone on the 13/01/2015. When I wanted to collect my new phone I was advised that the store did not have any stock and they did not know when they would receive stock. This was the only assurance I was given which is unacceptable. I have therefore opted to cancel the contract but am not getting assistance from both the store as well as the retentions centre whom are passing the buck and pointing me to one another for assistance. I have been in store more than 3 times and called the call centre the same amount of times. I do not think that it is fair that I am paying R499 a month and have nothing to show for it. I have been with MTN for a while and do not feel as though I would to continue my relationship with the network operator any longer.