This will let Three users access VoLTE via a new setting in the mobile data options. Not only that, but you can also turn off 4G and just have 3G if you wish. You can also have “Voice & Data” for VoLTE and “Data Only”, which basically carrier voice over the usual channels and keeps data on 4G only.
Turning on VoLTE via Three will mean that you’re rigged onto their 800MHz 4G network. At this lower frequency there’s better coverage because, as we all know, lower frequencies go further and tend to penetrate buildings better. If you’re on 2100MHz then you’ll get great speed and a higher-capacity signal, but not always great in-building coverage and it’ll not travel quite as far.