For some reason lots of Android phones (even the Pixel models from Google) seem to use a whitelist approach that is quite restrictive regarding where VoLTE is supported. For example at least up until recently (and probably still the case?) Pixel phones only supported VoLTE in countries that were in the list of countries in which the Google store official sold Pixel phones.

Meanwhile newer iOS versions seem to nowadays have generic VoLTE (and even VoWiFi) support that even works for smaller MVNOs without an iPhone carrier profile (as long as their VoLTE implementation somewhat conforms to standards).