Android
Change Default SMS App auf Android
When texts open in an app you rarely use, or new-message alerts don't show up at all, Android is likely still routing SMS traffic to the wrong app. This assignment is controlled centrally in the system settings, not inside the messaging apps themselves.
How the Default SMS App Setting Works
Only one app can register as the handler for incoming SMS and MMS at a time. Android keeps this assignment under Settings ➔ Apps ➔ Default apps ➔ SMS app, where you pick the app that should take over. Selecting a new app there immediately changes which one receives future texts and which one shows the SMS icon in notifications.
When Another Fix Makes More Sense
If messages disappear only inside one specific app, check that app's own notification permissions first instead of switching the default. Changing the default app does not delete existing conversations, but older texts usually stay locked inside the previous app if it stores them in its own local database.
RCS chat features, read receipts and group chat behavior can differ between apps. Test an existing group thread after switching before relying on the new default for daily use.