VoLTE works for me shortly.
After a reboot I need to activate it again.
I was in Luxembourg last weekend and had to activate VoLTE. It work directly, but as mentioned,
the setting did not stick after a reboot.
In my home country/area, VoLTE is not available, so I’m not using it.
I wonder if the bug @Dos has discoverd in the current modem is the reason for (maybe) switching to another brand.