Earlier quoted context omitted.
I think the sentence before that is more important though I do wonder if that is correct: In order to make a VoLTE phone call, your phone, your provider, the receiving phone and the intermediate network providers must all support the protocol. The receiving phone ? WTF? So I can't use my VoLTE phone to call a land-line in another country? That would be stupid if true. Meaning that really can't be true.
I'm pretty sure they're confusing VoLTE which only needs your phone and your provider to support it (and provision it, etc, etc, if you're roaming, there's more, etc, etc), and the higher fidelity "HD Voice" G.722.2 codec, which does require end to end support. Your provider might connect your end as HD Voice to a call terminating as POTS on the other end, but I'm guessing probably won't.
I have various old phones at family members' homes that still speak ISDN or "regular landline" to the respective router. The router used to also connect via ISDN and/or regular POTS to the phone company at various points in time. Nowadays it's actually some sort of VOIP protocol as well and those old phones don't care. Nor should they need to care.
Nor should a caller somewhere on the other end of the world care about that and vice versa what that caller uses to connect to their provider.
If VoLTE doesn't support that, then color me unimpressed and fully against it. Not that I can stop it, but it would be completely utterly stupid ;)