Iphone 11 pro silent button not turning off volume when receiving calls

Likely already familiar:

About the buttons and switches on your iPhone, iPad, or iPod touch - Apple Support

What to do If a button or switch isn't working on your iPhone or iPad - Apple Support

If a button or switch isn't working on your iPhone or iPad, here are a few steps that might fix the issue. If they don't, you'll need to set up a repair.

1. Remove any protective films or cases that might block the button or switch.
2. Check for debris or dirt around the button or switch and clean with a soft cloth.
3. If the button or switch still isn't working, is damaged, or works intermittently, you can set up a repair... {see article for links}