Unfortunately that is a network dependant feature and I was miffed to find out unless whatever network you use has to provision the use of data calling. I'm not sure if iPhone has the same issue although I've seen it working on Three and Vodafone without any issues.
It seems to be another way for networks to lock you into ownership or at least to ensure they use their network. Anyway hope you get it fixed one way or another.
Yes you will need a network connection for your sim to update but check first you are looking in the right place for your settings.....
If you can’t find the settings here, try going to Phone > tap the 3 dots in the top right corner > Settings > scroll down to find Wifi Calling and toggle On.
If the S7 behaves the same as my S9 on O2 you will only get WiFi calling if your phone can't latch on to any kind of O2 cellular signal at all. This can be tested by putting the phone into Flight Mode and then just enabling WiFi. To get back onto the O2 cellular network you would need to switch off Flight Mode.
Hope that helps.
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