The SOS icon means you're off your network, but you can still make emergency 911 calls. The phone will connect to whatever network is available (if any) to make the call. By law (in the U.S.), the other networks have to carry the call free of charge.
I have an iPhone X that's not signed up with any network, and isn't capable of satellite communications, but still displays the SOS icon because it can make 911 calls.
I'm talking about the SOS + satellite icons appearing together. In my experience, that only appears if there is no other network available, which is the case in most subway tunnels in NYC.
I have an iPhone X that's not signed up with any network, and isn't capable of satellite communications, but still displays the SOS icon because it can make 911 calls.