Explanation for Wi-Fi Calling

Hello,

I would like to have some explanation on how Wi-Fi calling is enabled on the d6603. My mobile operator (Salt Switzerland), has wi-fi calling service enabled, but on my d6603 there's not wi-fi calling option. For what I know, there are some operator, like T-Mobile, that enabled wi-fi calling on the d6603, so the phone is capable of using this feature.
My question is: wi-fi calling is an option of Android that should be enabled by rooting the phone, or is an option int the manufacturer's software?

Thanks

Edika