Even if MS fixes this, in the firmware (and any corresponding OS change), you need a carrier that supports it.
To-date, no carriers support VoLTE on non-branded Win Phones, in the US, that I'm aware of anyway. I suppose it's possible there's a sub-carrier that offers support, but certainly none of the larger ones.
Microsoft would likely have to initiate this with a carrier(s) as well.
It's a pretty big set of hurdles (unfortunately), and probably why it hasn't gone anywhere.
AT&T doesn't even allow any non-branded devices (except iOS) to use VoLTE, what-so-ever. My Nexus 6P, which is arguably one of the most VoLTE/WiFi calling capable devices out there, isn't supported either (I could use it on TMo though, if their rural coverage wasn't so poor anywhere near us).