If VoLTE didn't work after key combination and turning on "4G calls", it seems like You carrier didn't have settings for this phone or is blocking Your phone intentionally. Sadly, neither You or ASUS can't do anything with this. Asus can unlock VoLTE swith in menu, but after switching VoLTE stil won't work. I'll recommend change carrier, like me.
You all people complaining on Asus need to understand one thing - Asus is small brand in mobile phones, so they don't have as much power in negotiation like e.g. Samsung or Apple. When new model of Samsung or Apple arriving, carriers probably asking by themselfs on all settings for VoLTE, VoWiFi and 5G, because they wat to provide the best services. When new model of Asus arriving they didn't care, because it's probably less than 1% of their traffic.
The best thing, which We can do is actually buying these phones and massively asking on customer support of our carriers why VoLTE didn't work, and demanding to unlock. If there will be enough angry customers, carriers problably ask Asus for cooperation themselfs.
