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I understand from your post that you are not always receiving an audio alert for your incoming calls or messages. The following articles have steps which may help resolve this type of issue:
Use notifications on your iPhone, iPad, and iPod touch - Apple Support
If you don't see notifications for a specific app
To get notifications, connect to a Wi-Fi or cellular network. If you don't get notifications for a specific app, try these steps:
- Make sure that the app supports notifications. Go to Settings > Notifications, select the app, and make sure that Notifications are turned on.
- If you have notifications turned on for an app but you're not receiving alerts, you might have Banners turned off. Go to Settings > Notifications, turn on Show as Banners and choose a type of banner that you want to see.
- Make sure that you’re signed in to your Apple ID. Go to Settings > iTunes & App Stores and enter your Apple ID and password.
- Make sure that Do Not Disturb is turned off. Go to Settings > Do Not Disturb and turn it off. Or swipe up from the bottom of the screen to open Control Center and tap .
- If you’ve recently installed an app or restored from a backup, open the app to start getting notifications.
iPhone User Guide - Basics - Sounds and haptics
Set sound options. Go to Settings > Sounds & Haptics (on supported models) or Settings > Sounds (other models) for options such as ringtones and alert tones, vibration settings and patterns, and ringer and alert volumes.
Also, make sure that Do Not Disturb is not turned on.
Use Do Not Disturb on your iPhone, iPad, and iPod touch
Best Regards.