I just want to post a guide for people wondering how to enable VoLTE in unsupported countries. Apparently, Xiaomi only enable VoLTE for SEA countries etc. and Japan (Where I'm staying) is not supported.
So, I have been having problems receiving SMS and calling other numbers. After reading this blog post, I have solved that problem and maybe I can help others solving this problem. This is the procedure written in the blog post in English.
1. Setting APN (Might not be applicable for other countries)
for Au user with VoLTE enabled sim, create a new APN and set it as below
for DoCoMo user with VoLTE enabled sim, create a new APN and set it as below
2. Go to dial and dial *#*#86583#*#* . It should show carrier check disabled or something along the lines.
3. If the phone does not have a signal after step 2. reboot the device and it should connect after that.
Hope this will help anyone who is having trouble enabling VoLTE.