system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How are emergency services numbers still contactable by phone in areas with no signal?

In some areas you can't get a signal to call a normal landline/mobile number but there is the option for "emergency calls only". Are they on a different signal band?
gpt
All cell phones are required to always allow emergency calls if possible. If your normal network provider has no coverage but some other provider does then it will use the other network for the call. 

This requirement to always allow emergency calls also means that you can call 911 on a locked phone or one that is out of credit. If the phone is physically able to make the call it will.

You can even call 911 in many countries where 911 isn't the emergency number. The same goes for 999 and 112 too. That is done so a tourist in a panic can still dial the number they are used to calling rather than trying to remember the local one.