When there is no cellphone signal, how does the “emergency calls only” function works?

Question

When there is no cellphone signal, how does the “emergency calls only” function works?

In: 12

Answer 1

Because other providers’ signal is used if it is an emergency call.

Because other providers’ signal is used if it is an emergency call.

This is one of those things which has a simple answer and does not require a detailed “eli5” type response.