Hum… I just last week got Volte to work in France with Free on a 10 II, I’m not sure about the III but here was the steps for the II:
devel-su
cd /vendor/oem/modem-config/
mkdir S8683.1
cd S8683.1
echo “mcfg_sw/generic/common/default/vl/global/mcfg_sw.mbn” > modem.conf
You can follow it on that thread: