MTN not wanting to sort out the issue

My wife recently upgraded at MTN in Boksburg. Samsung Galaxy Core. The first phone had an issue - just switching off by itself, we took it back and was given the same phone, same issue occured, we where advised to take the phone to the samsung repair centre, they looked at the phone and was advised that it was a software problem. Phone did exactly the same after SSS cellular \repaired"the phone. We took it back to MTN boksburg"