Originally Posted by
serpens
Thank you to all the posters who explain IMS and other intricacies.
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?
Calls are made using VoLTE which does use data with the provider counting the calls in a different call bucket instead of data bucket. FYI, iMessages uses data which are not counted as SMS/MMS.
With your configuration, WiFi calling should be enabled. There are a few MVNO plans which do not offer WiFi calling or the plan requires a customer service request to enable WiFi calling (some Red Pocket plans). A FT member from Australia or New Zealand used a carrier which placed a geo location limit to wifi calling. I am not aware of any US provider with this limitation.
At one time I traveled with a 2nd phone with a local sim and connected my US iPhone by WiFi with the roaming data off. IMS eliminated my need for a 2nd phone.
In April, I will test my work Pixel with backup calling on my trip to Latvia. This is supposed to work the same as my iPhone setup after Google updated backup calling to use roaming. The Samsung S23 is supposed to have the same capability called DSDV as the iPhone and Pixel to avoid add on international plans.