Sonny Experia Z3 Compact

Never had a problem with any of my sonny mobiles . This time i got a mobile that has been advertise as water and dust resistance. Got home afternoon with flat battery and put mobile on charge. It went off and never back on again. Send it to mtn repair centre and got a sms after 3 weeks phone is BER. So why did i purchase the mobile with all the great stuff of water and dust bit still it gets liquid inside. Not verry happy with this. A mtn and sonny custumer of over 12 years. Not good.