score:4
Per Wikipedia, it looks like the Japanese government's J-Alert system uses SMS Cell Broadcast for notifications to mobile phones:
https://en.m.wikipedia.org/wiki/J-Alert
https://en.m.wikipedia.org/wiki/Cell_Broadcast
Basically, the messages are multicast to every single mobile device within a targeted area. This means that, if you have a Japanese SIM card (or, potentially, any or no SIM card at all) and are within a targeted mobile cell, you'll receive missile alerts, even if you have not signed up for them!
The catch is that because CB is completely indiscriminate, it's likely the alerts will only be in Japanese.
Upvote:1
Many countries have a service where you can register as a citizen who is currently overseas, and they will notify you about events that happen relevant to the part of the world you're currently in. For example:
I imagine the time taken to actually notify you of urgent events varies widely by country.