[Solutions are provided at bottom]
Here are some suggestions if your encountered VOLTE was disppearted/lost from phone.
[ims issue]
It is found that VOLTE function will be disappeared after system update. Acutally, when a NEW SIM card (not used in that phone) is inserted into a phone, the system should register the network and get network information (ims APN, etc). The system registers to IP Multimedia Subsystem if network is support. If everything fine, the VOLTE option will be showed in the network menu automatcally.
But authentication/other functions may be failed after system update due to incorrect information in the system. Therefore, the VOLTE option is not showed.
To solve the issues, you can't simply clean network information from menu or clean network process data and cache. All are not working.
**A reminder: Not a problem, but do switch off the phone before remove / insert SIM card**
[Solutions]
1) SWAP SIM card
If the Xperia has two slots and the SIM card never be inserted into another slot. Try to insert the SIM into slot 2, the system should start the whole SIM registration (include ims) process (reboot is required by Xperia).
If the SIM is not locked by carrier, you can try to do precheck before apply following solutions.
Borrow someone SIM card (should not be used by your Xperia before) to ensure the phone encountered the ims issue or not. Once the “new” SIM card insert to Xperia, after Xperia start up the system should request to restart again. If VOLTE option is appeared again, you can sure you can apply following solution.
2) Clean system information
If you are not sure the cause of issue, you can try this anyway.
*******Warning*******
The system/app ID/PASSWORD OR information (e.g. google ID/PASS, facbook ID/PASS ….) and some applications data will be lost.
a) Manual backup all information to SD card under system menu
b) **Take out the SIM card**
c) Factory reset under system menu
d) **Complete the setup process but SKIP/NOT login with your Google account
e) **Insert the SIM
f) **Check the ims APN is downloaded or not (if not download again or add it manually)
g) The VOLTE should be appeared again
h) **Add back your Google account
i) **Restore all information but DON’T restore device setting & network information.
3) Ask carirrer to provide a new SIM for your if you sure encountered missing VOLTE issue
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Once the issue was solved. Do a manual backup and keep the image separately
Disable the auto system update.
PS: Some users reported that the VOLTE is show again after upgrade to android 9
