Thank you very much for this info.VOLTE FIX [only for Android R latest builds]
• Go to Settings > PHH Settings > IMS Features
• Now select both Request IMS Network and Force Presence of 4G Calling
• Now click Install IMS APK (for Mediatek or Qualcomm vendor)
• Let it complete download (check your notification panel). It will show IMS installed, you may now reboot
• Now click Create IMS APN (nothing happen)
• Now Make sure to enable VoLTE icon toggle in Customization > Status Bar > System icons (not in all ROMs)
• Now Reboot!
• After succesfull reboot, you will get VoLTE Toggle in Settings > Mobile Network
• Now try to re-enable VoLTE in Settings > Mobile Networks
NOTE: don't work with Exynos and Samsung phones
Let me try this.