[gst]

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).

[supertrope]

There are a lot of interoperability bugs in VoLTE. The best part of standards is that each vendor implements it slightly differently.

Apple tends to hold back a feature until all carriers have blessed it. Apple withholds Carrier Bundles from MVNOs who don’t promote iPhones, so iPhones are more likely to be used on fully compatible carriers. They also have significant market weight so a MNO is less likely to decide they can’t bother to fix interoperability bugs with iPhones.

There’s a correlation between buyers of expensive smartphones and higher ARPU customers which further increases incentive to put in the engineering resources to squash bugs but only for flagship phones. MNOs also have an incentive against improving interoperability as it makes it easier to switch to another carrier (“churn”).

[kiwijamo]

And worse, it differs when roaming. I am aware there are networks where if you are their customer VoLTE works but if you are roaming onto their network it does not work at all. With the exact same device.

[dangus]

I’d have to think that a network that shuts down their 2G network probably allows roaming devices to make calls over VoLTE. All those carriers that don’t allow roaming devices to make VoLTE calls probably have an active 2G network.