To be clear, most of our devices are indeed VoLTE compatible when it comes to hardware & bands etc…
However, most cellular providers have standards (config files) that need to be met to enable VoLTE to work.
LTE data is no problem.
There is also the fact that your watch may appear to be using 4G for calls - but in reality it is switching to 3G automatically for voice calls.
You can test this by selecting 4G only in the watch engineering mode settings (research if you are unfamiliar with this).
If you set to 4G /VoLTE only for sim card and you cannot make voice calls - this is why. Your sim provider is not compatible with these watches.
Please take time to understand what this might mean for you.
We are researching what settings (configuration changes) are needed to get VoLTE to work, but it’s a big project and feedback is needed.
Cheers
Pablo11.