nztim:
VoLTE requires a whole heap of settings programmed into the phone to work (for example the carriers call server/voice protocols/etc) worldwide carrier profiles are all programmed into any iPhone 6s and newer along with almost every 4G frequency, and, with the 12 and above almost every 5G frequency
It is the Multiple manufacturers of android phones who have decided to cherry pick which profiles they have decided to put into various models of phone, and that is no fault of the carriers who have made this information available to phone manufacturers.
Visitors to NZ with iPhones that are 6s or newer will have no problem connect to the networks here, those with Androids will be hit and miss
AT&T in the USA have led the way here, by putting together a full-scale PDF of the various phone models (and sub models) which manufacturers have decided to include their VoLTE profile
https://www.att.com/scmsassets/support/wireless/devices-working-on-att-network.pdf
Any reason they couldn't have designed it so that the VoLTE profiles get pulled down over the cellular network? Having to have the profiles baked into the firmware/OS seems rather dumb, and the outcome rather foreseeable.