VoLTE requires carrier support unless some very clever hacks I'm unaware. I am pretty sure that's the case with PinePhone as well, meaning it will drop to 3G.
I assume they want to support it, as well as any other carrier feature. I just have no knowledge on how difficult it is, nor in what state it is in now.
https://www.pine64.org/2019/12/05/december-update-thank-you-...