Country: FR
Provider: RED By SFR
Device: Xperia 10 II (XQ-AU52)
Registered: ✓
Voice in: ✓
Voice out: ✓
SMS in: ✓
SMS out: ✓
In order to have it work you need to copy the modem config file of SFR (S12.1) to RED By SFR (S6.3)
devel-su
mkdir /vendor/oem/modem-config/S6.3/
cp /vendor/oem/modem-config/S12.1/modem.conf /vendor/oem/modem-config/S6.3/modem.conf
Reboot the device and voila the VoLTE work!