I have been asking the same.. There are a few good mi 6 ROMs on the forum but I haven't found a WiFi calling one. Its encoded into your build. I don't know but maybe one of the devs on here would be kind enough to tell me how to repack a ROM the files are on loads of other aosp and lineage os devices. I'm sure a simple zip flash should do it if correct files available.
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Here is a link to the aosp open source WiFi calling source code link.
PS. There's an android.mk file there which seems to be a make file but traditional Linux commands won't work. Does anyone know how to do a post install of WiFi calling. A little bit of kindness goes a long way.
That's the latest build he has released and don't forget to format your internal storage. It is encrypted and will put you in a boot loop. Follow his instructions and it will work a treat. The link above has full gapps.. Flash the small miui firmware kernel first to let it work. Any questions or stuck inbox me so the developer isn't inundated with questions.