You can reliably show scheduled reminders on iOS when the app is killed - but with important limitations. The only fully-supported mechanisms are:

• Local notifications scheduled with the system (UNUserNotificationCenter). They will be delivered when the app is terminated, but the notification sound is limited (system-truncated to ~30s) and normal notifications respect mute/Do Not Disturb.

• Remote (APNs) notifications from your server can also wake the device and play a sound (again: subject to delivery delays and to the same sound limitations unless you have Critical Alerts).

• If you need to override mute / DND or play an alarm sound longer than the standard notification sound, you must request and be granted the Critical Alerts entitlement from Apple — otherwise you can’t reproduce the Clock app behavior exactly.

What isn’t allowed / realistic:

• PushKit (VoIP) / CallKit workarounds for non-VoIP alarms are not acceptable — Apple restricts PushKit for VoIP and will reject apps abusing it.

• Playing continuous audio while the app is terminated is not possible unless the app is running with background-audio mode (which doesn’t help if the app was force-killed). Apps don’t have the same system privileges as Apple’s built-in Clock app.

Which approach you should pick (practical guidance)

1. If “notification + short sound / vibration” is enough (typical attendance reminder)

   • Use flutter_local_notifications (or schedule using native iOS API via platform channel) so the system schedules the notification. Make sure the sound file is bundled with the app and ≤ 30 seconds. The system will deliver the scheduled notification even if your app is killed. This is the simplest and works on both Android and iOS.

2. If you need higher reliability / server control / remote scheduling

   • Use a server to schedule an APNs push at the required time (payload includes sound). That removes dependency on the app scheduling locally and lets you change reminders from the server. Delivery timing is not guaranteed to the second, and network/APNs limitations apply. (Still subject to sound/DND limits unless you have Critical Alerts.)

3. If you need to override mute/DND or play long/continuous alarm sound

   • Apply for the Critical Alerts entitlement from Apple (requires a strong justification). With this you can play sounds even in silent/DND and may get more consistent alarm behavior. Getting the entitlement requires App Review and a special entitlement and provisioning.

4. DO NOT try to use PushKit/VoIP unless your app truly is a VoIP app - Apple enforces this and will reject misuse