Apple India — iphone x calling failed

Address: Kolkata, West Bengal, 700055

My biggest mistake is to take iphone x.i have used this phone but repeatedly my calling is failed, i can not ever talk to any one because some how phone calling problem is there..i have complained this to authorised service centre systematix media in kolkata (city centre-1) complain no SL/13616..but they are taking phone for 6-7 days and then they tell it will now defenitely work..but same this is happending repetedily..kindly replace phone