The short answer
Yes. Alarms are scheduled locally on the device, so airplane mode, no SIM and no Wi-Fi all leave them working normally on both iPhone and Android. The exception is third-party alarm apps that fetch something — ads, config or sounds — at the moment the alarm fires.
Why alarms survive airplane mode
An alarm is a local timer. When you save one, the operating system records the time and wakes the device itself. No server is involved, which is why alarms have always worked on planes, in basements, and on phones with no SIM.
When an alarm app breaks this
A third-party alarm app can undo that guarantee, and some do. Watch for three patterns.
- Advertising shown on the ring or dismiss screen. If the ad has to load before you can silence the alarm, no network means a stuck screen.
- Cloud features on the dismiss path — remote puzzle generation, cloud photo matching, or an account check.
- Sounds streamed rather than bundled, including music-service integrations that need to reach a server.
Roosta treats this as a hard rule: nothing on the alarm path requires a network. Alarms are scheduled by the operating system, all sounds ship with the app, and camera missions run their recognition on the device. A user with no signal still wakes up, and every mission still works.