O2 UK finally joins the VoLTE and wifi calling club

Years after many of its competitors had already done so, O2 UK has finally switched on VoLTE (naming it 4G calling) and wifi calling.
The accompanying propaganda is pretty standard: VoLTE provides clearer calls and better indoor coverage, while wifi calling provides an alternative source of connectivity in cellular not-spots. Initially, however, both services will only work on eligible iPhone 7 phones, with other devices following in due course.
This is, at least, proper wifi calling, as opposed