rocknrollait depends both by operator and by software/hw.
Thanks Anders. I contacted ASUS Spain support, but as I expected, they had no idea what I was talking about. After explaining to the support guy, he finally told me they didn't have any information about this functionality in future updates.
So... it looks I will be stuck with normal 3G calling with the Zenfone 6...
Which brings up another issue: with this phone, I haven't been able to make any 3G calls with "HD" codec, while if I put my SIM in an older ASUS Zenfone 3 Max, during a call it shows the HD icon next to the call duration and it can be clearly heard the call quality is way better. The same happens with another cheap phone with vanilla Android 8 that I have tested with.
Why isn't this working in the Zenfone 6? One would expect at least HD 3G calls in a recent phone like this. Is it region dependent? Can it be enabled somehow?