How sure of that are you? Have you tested it? I mean, I'm not necessarily saying that you're wrong, but for that to be true, VoLTE would have to be turned on by default and operating in the way it was designed... and in my (admittedly limited) experience, that's not necessarily always a guarantee: when I tested it out awhile back on my Sprint iPhone X (which is most assuredly an LTE phone) things didn't function quite as expected, regardless of whether or not VoLTE was enabled.
That said, I'll admit that test was awhile ago... possibly over a year, now. Maybe I'll turn VoLTE back on again this evening, and see if things have changed.