My iPhone says SOS in top right corner
[Re-Titled by Moderator]
iPhone 12 Pro Max
If you see No Service or Searching in the status bar on your iPhone or iPad (Wi-Fi + Cellular), your device isn't connected to a mobile network. If you see SOS or "SOS only" in the status bar, your device can still make emergency calls.
If you see No Service, Searching or SOS on your iPhone or iPad