2 reasons that it will not show up in your SIM settings. 1) The phone manufacturer can disabled the feature in the firmware example Asus, some Motorola phones, etc. 2) The carrier whitelisted the phone and disabled the feature in their system which means the SIM will not allow WiFi calling on a phone. AT&T only allows WiFi calling on their phones only and any other unlocked phone will not have this feature only HD calling aka VoLTE calling. T-Mobile at least in the U.S. will let unlocked phones with WiFi calling ability work with their SIM cards. WiFi calling at least with T-Mobile in the U.S. will activate the feature once the account and SIM are established, but other carriers may not activate the feature (if available) then the end user must turn it on thru the SIM settings on the phone (if it is available in the menu in the first place meaning you will see the feature as an option in the SIM settings, and if it is not shown in the SIM menu then you are out of luck). Most phones like Nokia, OnePlus, and some Motorola phones allow WiFi calling and VoLTE hence don't block these features in their firmware, but the carrier can disabled these features tho in their system via their SIM card meaning even if a phone can do these features the carrier has ultimate control over the feature usage.
