I will qoute Anders on what he said for VoLTE.
"It's really complicated and here's why
Some carriers charge a lot of money for VoLTE support. Some will do it for free. But in both cases, you need extensive field testing. Even when the carrier do cooperate, it often involves a lot of meetings, documents going back and forth, to make sure compatibility is good enough. In some cases asus can do the field testing and in other cases the carrier want to do it themselves. But this doesn't matter that our phones will have high priority. It can takes months and months before the testing is done.
It's much easier for bigger brands that are usually sold by the carriers themselves. It's also a lot easier getting support if it's a model with huge sales volume since it will mean more customers for the carrier. We're not there yet which means that we're not a priority for most carriers.
Some brands have partly solved this by allowing users to activate unsupported VoLTE. I may work but there's no guarantee just because the hardware is there. It could also become a daily issues for the customer and in rare cases it could even become a legal issue for the manufacturer. An example would be where the customer is in a life threatening situation and can't call anyone because the phone tries to make the call via VoLTE. Who has the responsibility if someone dies? This is a worst case (what if) scenario but it still needs to be considered.
You can also imagine that the relationship between the manufacturer that allows unsupported VoLTE and the carrier wouldn't exactly improve if this carrier receives a lot of complaints from customers using unsupported VoLTE. You may remember today and next week that you activated VoLTE without official support in a hidden menu but eventually most people forget and when important phone calls can't be made, someone is to blame. That angry phone call, email or forum thread will go to either the carrier or the manufacturer as we humans (in general) are very good at blaming everything and everyone but ourselves."