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Fix VoLTE in native Pie ROMs manually:
1. Flash desired Pie ROM and addons (Gapps and Magisk generally).
2. Go to mount section in TWRP mount vendor (tick it) and untick read-only.
3) In the vendor folder, /pull the build.prop and transfer to a PC if available, or else send it to an SD card using the default TWRP file explorer and insert into any other rooted/unrooted device, then edit the line ‘service.qti.blahh’ into ‘vendor.service.qti’.
4. Send it back to rosy (either way, if you edited using PC, send directly, if you edited using a secondary device, send it into an sd card and insert it into rosy and transfer using default TWRP file explorer.
4. Give chmod 644 permission for the vendor/build, whichever variant of TWRP you have, just give permissions in terminal as mentioned.
5. Restart the device, VoLTE should be working.
Thanks to @mOnUuuuuu for the guide.
Thanks for editing.