Apple India — phone heated too much when use net or music

My phone always change it self too brightness when i shoud adjust manually and its too heated compared other i don’t know why iphone change this type and it also when any one cal to me its doesn’t show order wise.. I already use old iphone 6 (16gb) thats too good.. But it was really soo many problems.. Soo am going too iphone center to exchange it..