I've just bought myself a Moto G6, after I broke my old phone (installing LineageOS).
I've noticed that on many/most occasions, when I call either of my children (who have an iPhone 6s and iPhone 7), the voice quality is astounding - it sounds almost CD quality, compared to the usual muffled call quality when I call my wife's iPhone 4S or a standard land-line.
I've done a small amount of searching, and as of this time last year, there was no sign of Plusnet (or any other EE MVNO) being allowed to use VoLTE.
Is this still the case? If it is the case, why/how are the calls coming through sounding so good?
We don't currently support VoLTE and have no plans to implement this at present.
I suspect this is something to do with advanced microphone technology on the newer models of iPhone - if I remember correctly, Apple now include two microphones on the iPhone and better technology to reduce ambient noise.