Wi-Fi calling option not showing

I’m temporarily using an IPhone 6 Plus and because I live in an area which doesn’t have much network coverage I require Wi-Fi calling

it is on iOS 12.4.7 I think or whatever is the latest available update for iPhones 6 plus

when I try turning it on it doesn’t show the option of Wi-Fi calling

i use airtel which supports and provides Wi-Fi calling as other members of family use it to but on higher iPhones models