After many days of search and trials I have managed VoLTE to enable it in Xperia Z2 & Z3 in Android 6.0.1 (23.5.A.1.291)
Only For Xperia Z2 (D6503), Xperia Z3 (D6653) & Xperia Z3 (D6603)
Your carrier must support VoLTE service and it should not be locked to certain devices by your carrier.
This method Only works on stock roms.
Tested and working !
Operator- Reliance Jio, India.
Thanks to @jitz975 @Dilesh Perera @kevev @tititoto
Try at your own risk.
Always make a backup of system and data before doing this in case anything goes wrong
Follow these steps,
1. Root Access and Recovery are mandatory.
2. Download & flash VoLTE .zip via recovery.
3. Find this line in /system/build.prop [ro.build.type=user] & change it to [ro.build.type=userdebug] then restart phone.
4. Then u have to find the modem switcher.
its in system/priv-app and its called CustomizationSelector.apk
> download Quick shortcut maker from play store
> open Quick shortcut maker to make a shortcut using the app then go to customisation Selector \ modem switcher
> open Customization Selector app and select modem according to your network operator.
> if u dont find your operator just select amss_fsg_volte_base_operator_trials_tar.
> in my case Z2 D6503 i have selected amss_fsg_Reliance_jio_tar.
> try other modems if any of these wont work.
> set display density to 480 [ ro.sf.lcd_density=480 ] from build.prop if Customization Selector app doesn't open.
> wipe dalvik/cache if Customization Selector app doesn't open.
> if it wont work even after wiping cache then try another stock Android 6.0.1 (23.5.A.1.291) rom from xperifirm
5.. After selecting modem, phone will restart and you are good to go.