When your iPhone displays "SOS, " it's indicating that it is not connected to the usual cellular network but can still make emergency calls. Typically, this situation arises when you are in an area with a very weak or no signal, there is a problem with your SIM card, your carrier service is down, or Airplane Mode was recently turned on. One of these solutions might help you: move to a place where the signal is better, restart your phone, turn Airplane Mode on and off, or check if your SIM card is properly inserted.