3rd phone stopped working in 4 months

I got a new Iphone 6 plus on the 8 DEC 2014. This was part of my upgrade on my contract. Towards the end of February 2015 my new phone stopped working, just went went dead. I went to MTN repair centre in Midrand on the 28 February 2015, and was given a brand new iPhone 6plus 64, this took me 2 hours 30 minutes!!. On the 25 March 2015, while on a call, the new phone once again just stopped working so went back to MTN repair and got given another new iPhone 6plus 64, with no explanation as to why the previous one wasn't working. Today on the 20 April 2015 my 3rd new phone once again stopped working. I have had 3 Iphones since Dec that have all just stopped working. I phone head office and they don't even bother to answer their phones, phone customer repairs and they can't assist. No one at MTN seems to think this is an issue. I went to the MTN branch nearest me and they say it is an Apple issue. How is that possible as my contract is with MTN not Apple. I asked Apple why the iPhone 6plus is causing issues and they said that MTN received a batch of phones that has faulty software All i want is a phone that works and why should i be sitting for over 2 hours in the repair centre!!