Sony Xperia M5 faulty

I purchased a Sony Xperia M5. After a week or so the phone has begun shutting down for no reason and the only way to get it back on is to plug it in. This happens regardless of the battery level. Sony say they have been aware of this problem for a while and they suspect it is a software issue but they do not yet have a solution. They have suggested that I take it back to MTN where a bought it and get it replaced. I don't think that is fair that I must be inconvenienced by a poor Sony product. Sony should be offering to bring me a new phone and they can sort it out with MTN. So if you are considering buying a Sony Xperia M5 or any other Sony Xperia, don't.<br> Call ref no ***. Please get someone with the technical expertise to fix this to contact me or bring me a new phone. That is customer service.