On the latest iPhones, SOS mode is the emergency fallback to satellite service. It's really meant to be used in situations where you're well outside of any sort of service area but you have a clear, unobstructed view of the sky.
Your iPhone will instruct you on where to point and help you track an emergency satellite that is manned by live humans who will take your emergency request and relay it to the proper people.
I’m pretty sure that “SOS only” can also mean the phone seeing networks it can’t register with but which it could make an emergency call on if required. This predates satellite SOS.
Your iPhone will instruct you on where to point and help you track an emergency satellite that is manned by live humans who will take your emergency request and relay it to the proper people.
More specific info here: https://support.apple.com/en-us/104992