system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do calls to emergency numbers like 911 get placed even without a sim card?
gpt
A SIM card just has the credentials that you need to connect with your network.  Its basically ID for your phone so the tower knows who you are, what your phone number is, what carrier handles your account, etc.  

Even without this information, your phone can still see and connect to the tower; the tower _has_ to accept the connection because that is how the SIM data is normally passed on (right after connection).  In normal circumstances, the tower simply rejects your attempt to call without the SIM information - it basically says "No ID, no phone calls."

However, if you dial 911 the tower will accept that call no questions asked.