Ridiculously poor service

Bought a Samsung j110ace at mtn maponya mall for my mother on 28/11/2015. The phone phone was faulty from the word go. Took it back to the store and was told it might just need software update. The Samsung guy in store updated the software there was still no change. So my mom had to take it back to either be given a new phone or money back. Was told she would get another phone in 7days,and that was on 7/12/15. Called last Tuesday was told to come get the phone. My mother went the next day and was told theres no stock. That was the 3rd waste of the trip costing us more money. Called earlier today no stock still. Spoke to the manger Kennedy and requested for my money back and said he will call back in 15 minutes. Still waiting for my phone to ring. I'm ****** annoyed at been given the run around about something I paid in cash for. Bought screen protector and pouch for that phone which is now a waste as well. Can someone from MTN PROVIDE ME WITH DECENT SERVICE. I AM A PAYING CUSTOMER SHOW SOME RESPECT.