Sony M5 faulty

I renew my contract with MTN, I choose a Sony M5, and after a month the phone started to switch off by itself, I was in the middle of a conversation the phone was fully charge and it switched off, It s no way we could switch the phone back on, only if you plague it it will re start.<br> Apparently this phone come faulty, but they don't let you know.. so you have to be stuck without phone for 2 or 3 weeks, very unfair and very disappointed. <br> I cant be without phone, and if they new the phone have a problem they supposed to exchanged it straightaway.<br> <br> Please help<br> <br> Kind Regards<br> Patricia