SOS on an iPhone indicates that the device can only make emergency calls, as it not connected to the regular network. This happens when there....
https://lifpii.com/what-does-sos-mean-on-iphone/

“SOS Only” on an iPhone means the device is unable to connect to your regular cellular network but can still make emergency calls. This typically....
https://community.wd.com/t/what-does-sos-only-mean-on-iphone/295945