ELI5 Emergency Calls Only - If there is no cellphone signal, how does "emergency calls only" mode work?

If there is no cellphone signal, how does the "emergency calls only" mode works? How does the cell tower system coordinate to make this possible?   ... We explain like i'm five!   Thank you to the r/explainlikeimfive community, and specifically the following reddit users whose questions and responses formed the basis of this conversation: dcardeno, not_anonymouse, Pogoyoyo1 and billythewolfde.   If you are a contributor and would like to be a guest on an upcoming episode of this podcast, please reach out to us at [email protected].