Not taking responsibility for design defect

I purchased a Sony Xperia Z3 Compact on an upgrade contract through Vodacome 4U on Saturday 14 Feb 2015. On Monday 16 Feb 2015, after having the phone for just 2 days, the whole back glass \splintered"in my hand while I was walking. The phone has never been dropped by me (only had the dam thing for 2 days). I immediately took the phone back to Vodacom 4U (note within 7 days of purchase) and they told me that they needed to send it to Sony to be assessed. Today I get a call telling me that I need to pay for the repairs on the phone. Unacceptable. The phone was not damaged due to my negligence. I have subsequently been onto countless forums and have seen that ghost cracking of both the back and front glass of this phone is a very common problem and often happens when the phone is simply lying on a table or being charged and Sony continues to deny that there is a design default. This is unacceptable and I am very unhappy with Sony and the service / attitude of both Vodacom 4U and Sony. I would recommend anyone reading this"