Xamarin local notifications set to low importance still risk audible alerts on some devices

To achieve truly silent notifications that reside only in the notification shade or center, you must combine channel importance with explicit attribute suppression on Android and delegate-level presentation options on iOS.

Android: Overcoming OEM Overrides

While ImportanceLow or ImportanceMin generally suppresses sound and banners, some OEM skins (such as MIUI or EMUI) may apply their own defaults. To maximize reliability, you should explicitly disable sound and vibration on the NotificationChannel object during creation.

Implementation Strategy:

• Set Importance to ImportanceLow (no sound, no pop-up) or ImportanceMin (no sound, no status bar icon).
• Explicitly call SetSound(null, null) and SetVibrate(false).
• Critical Note: Notification channels are immutable once created. If you have already deployed a channel with a specific ID, changes to importance or sound will not take effect. You must either delete the channel or use a new unique channel ID.

iOS: Controlling Delivery Presentation

Requesting alert authorization at launch does not make silent delivery impossible. Authorization grants the capability to show alerts, but the UNUserNotificationCenterDelegate determines how a specific notification is presented when the app is in the foreground.

Implementation Strategy:

• Implement the WillPresentNotification method in your UNUserNotificationCenterDelegate.
• Return UNNotificationPresentationOption.None to ensure the notification lands in the Notification Center without triggering a banner, sound, or badge while the app is active.
• For background delivery, the system relies on the payload's sound and alert keys; omitting these or setting them to null ensures a silent arrival.

Practical Verification

Emulators often use a "clean" version of the OS that adheres strictly to API specifications. To verify behavior on physical devices:

1. Android: Navigate to Settings > Apps > [Your App] > Notifications. Check the specific channel to see if the OS has assigned it a "Silent" or "Low" priority. Test on at least one Pixel (Stock) and one Samsung or Xiaomi device to check for OEM overrides.
2. iOS: Test the transition between foreground and background. Verify that the UNNotificationPresentationOption.None logic triggers when the app is open, and that system-level settings in Settings > Notifications are not overriding your intent.

Missing Diagnostic Detail: Are you using a third-party push notification wrapper (like Firebase or Azure Notification Hubs) for these local notifications, or are you using the native NotificationCompat and UNUserNotificationCenter APIs directly? Wrappers can sometimes inject default sound payloads that override local settings.