Thank you to all the posters who explain IMS and other intricacies.
Originally Posted by
draver
[...] It is dependent upon your voice line NOT having roaming service. [...]
Is voice roaming independent of data roaming? FWIW, my MVNO (Consumer Cellular) only offers international roaming by request (and it wasn't clear if that was voice or data or both), and I have not requested it.
With my iPhone, what I have done in the past is turn off data roaming for my voice line (in Settings > Cellular > [MVNO] > Data Roaming) but leave Turn On This Line enabled on the same Settings page. While out of USA, I have an eSIM (usually Airalo) with data and roaming enabled. (Cellular Data is enabled by choosing the provider in Settings > Cellular > Cellular Data and roaming is enabled in Settings > Cellular > [eSIM provider] > Data Roaming.) It seems to work, although I made few calls and do not recall receiving a call with this setup. Have I just been lucky? Am I possibly missing calls?