Notification are no longer working on my iPhone 13 mini

My mother-in-law has a 2 year old iPhone 13 mini and recently text message and email notificataions and sounds have stopped working, although if you unlock the phone a badge is showing on the messages app. Also, when someone calls the sound is loud for about 1 second and then decreases to about half the volume. 

I've checked notification settings and they're all showing as set for banners on lock screen and notification centre etc. I've turned focus off and that makes no difference. I've turned the apple watch off and that makes no difference. I've tried turning imessage on/off, restarting the phone, soft resetting the phone and nothing fixes it. 

Notifications show up 'properly' on her iPad, turning the iPad off doesn't make any difference to the phone. Whatsapp messages work as expected, ie show banners on the lock screen and makes a sound. 

Does anyone have any idea what the issue is and how to fix it please? I don't know what iOS it's running but she normally keeps it up to date. 

[Re-Titled by Moderator]