Apple version is the latest 15.6. 1.
1. First find Apple's settings and open them (as shown below):
2. Continue to click "Sound and Touch" (as shown in the figure below):
3. Inside, you can see a "button adjustment" switch. If this switch is turned off, the volume button of the mobile phone cannot change the volume, and then we can adjust the volume in detail to affect the volume of message notification (as shown below):