SONY XPERIA Z3

MTN sold me the phone and informed me the phone was waterproof as long as covers closed. When the phone was wet it stopped operating. The covers were closed as the phone has a cover on to keep them closed. When I returned the phone to MTN they said that SONY should not advertise the phone is waterproof as many are returned and they do not agree with this advertising. I got drilled about the covers and explained they were closed and maybe there was an another issue or it leaked via the camera or other openings. the MTN staff were adamant not to return it saying it wont be replaced due to box or missing ear pieces. I am very irritated as this phone is 4 months old and if the phone is not waterproof then it is false advertising. I have retruned the phone and suspect I will be billed for repairs instead of new phone.