What happens when the mobile network goes down?

A mobile outage rarely takes out everything at once. Understanding which layer failed tells you what still works.

If the radio network is up but the core is down, your phone will show bars and refuse to connect. Emergency calls usually still route via any available network.

If a mast loses power or backhaul, coverage disappears locally. Neighbouring cells pick up some load, which is why you sometimes get signal by moving a few streets.

Wi-Fi calling is the most useful fallback most people already have, provided their broadband is on a different provider path.

For anyone who depends on being reachable, the practical answer is a second SIM on a different host network, kept in a drawer and topped up once a year.