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

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”).