This is a guide on how to enable VoLTE and Wi-Fi calling on the Mi 9 and (possibly) other MIUI 11 devices.
NOTE: Wi-Fi calling WILL NOT work if your carrier doesn't support it.
Root is required for any of this to work.
Download the Wi-Fi calling document because you will need the text inside
Go ahead and download build.prop editor by JRummy Apps from the play store. Open the app, allow superuser access, more permissions, blah blah blah whatever.
Now, press the pencil (that means edit the document) and scroll all the way to the bottom. Right at the end, go down a line and paste the text from the document. Now press save (floppy disk icon). Refer to image 1 if you had a hard time understanding this.
After doing this, restart your phone.
Now, go to settings, tap on SIM Cards and Network, tap on your (main) SIM card, and an option should be visible and should be asking you if you want to use WLAN to make calls. Image 2 should tell you what it should look like
Now to make sure that Wi-Fi calling and VoLTE is on, go to settings again, tap on My Device, tap on All Specs and tap on Internal Storage several times until a network testing menu appears. Tap on phone Information 1 (or wherever your main SIM card is on) and find an option called Mobile radio Tower. If you are having a hard time finding that, look at image 3. Wi-Fi calling, video calling, and VoLTE should all be turned on.
The VoLTE option sometimes doesn't turn on. To fix that, you just need to turn off the Mobile radio Tower option off, wait for it to slide to "off" and wait for a couple of seconds. Turn it back on and wait until the VoLTE option has slid to "on".
If all three are working, congrats! You have Wi-Fi calling and VoLTE!
Hope this helps!!
