Samsung S5

I purchased a brand new Samsung S5 at Makro in Port Elizabeth on 09 April 2015.After two weeks I encountered problems with the camera and insisted that the handset be replaced.Siyabonga phoned me ensured that I receive a new handset and in May I was contacted to collect a new handset.After a month I encountered different issues with the second handset.I was missing calls the screen would freeze up and scroll randomly without me touching it.Once again I booked it in and today 11/09/2015 received a call that the handset was ready for collection.The lady informed me that they had inserted a cell c simcard and everything was in order???What happens if i use my MTN simcard again?What is wrong with the latest Samsung handsets I had a S2 and not once gave me any hassles.What should I do?