Unfortunately, you're unlikely to hear a true VoLTE call soon, because they only work between two VoLTE phones that have both opted in on the same carrier. (That's why we don't test it, as we typically test call quality against land lines so we have a stable benchmark.) VoLTE isn't activated by default even on most phones that support it, and it is, right now, totally not interoperable. If a call needs to be handed to another carrier or a land line, it drops to the lowest common denominator of quality.
That deeply damages the VoLTE experience, and the HD calling experience in general. Verizon and AT&T will only deliver HD voice calling between two VoLTE phones, on the same network, with VoLTE turned on. Right now that's not that many phones. At least T-Mobile and Sprint have some form of HD voice quality to fall back on. But although all four carriers now support HD voice calling to some extent, you can't make an HD voice call between them.