If your carrier supports VoLTE/VoWiFi but not on your device you can easily enable it with some modifications. There is even a Magisk module to forcefully enable.
To enable VoLTE/VoWifi you need to add below lines in your
# Debug Options
# Radio Options
How to edit your build.prop file
All you need to edit the build.prop file is a text editor, and a file browser that can access the root directory. Most of these file browsers come with an integrated text editor so you don’t have to think about downloading one just to edit the build.prop file if you don’t already have a favorite editor enabled. Root Explorer, File Explorer Root Browser, and Solid Explorer are some common file browsers capable of doing just about anything a file browser could do. If you have another favorite file browser please let us know in the comments.
When you have a root enabled file browser, navigate to the /system/ folder and scroll down to find the build.prop file. Tap on it and choose ES Note Editor or any other editor installed on the device that you wish to use.
Move the original file into a separate folder and rename it as build.prop.bak before we change the build.prop file and then transfer this file back to the /system/ folder. This way, if anything goes wrong you don’t have to worry, because your system fails to boot. You should delete the changed build.prop from TWRP recovery, and remove the.bak from the name of the backup build.prop. Again your system is booting properly.