I have an iPhone SE, which I bought years ago direct from Apple. It has been used on various networks over the years, and is now with EE.
I am on a rolling 30-day contract, and have confirmed that VoLTE is working as expected.
My question is if I need to do anything to the phone to allow it to access the band 20 / 800MHz coverage. I've seen people mention needing EE specific firmware for other types of phone, but obviously the iPhone doesn't quite work in this way.