And are you actually in poor coverage to trigger WiFI Calling? 

This is a network level setting so if you're not in that magic RSRP range on the One NZ network it will not engage.

Not sure if this is the same issue, but I had the same problem on Mighty Mobile after switching to a new eSIM a few months back where VoLTE and WiFi Calling just stopped working.

Ended up having to escalate it and was resolved (after a lot of pushback), no idea what the issue was, but test VoLTE as well and make sure that works and if not, also mention it on your ticket.