Lots of problems

Hi I buy iPhone 12 Pro Max on 12 February and in few days lots of problems come ,first when I talk in what app ,normal call the voice is break lips are move but not not go in facetime whatapp , without do anythink screen become white and grey, sometime phone become hang I go on 6 April their check all Fone and said now we check it ok if u have any think wrong please come back we replace phone or what u want we do for you I m happy and on 8 April App Store are missed when we again go first I show lot App Store is not able he see and check App Store back I buy new phone and why I go again and again for check ,their see and he said yes App Store are missed, I am not happy their is any problem IN phone please refund me money back