Iphone 12 pro max
When incoming calls my ring tone starts loud and slow down by it self even if adjust to high on the settings(hard to hear)
iPhone 11 Pro Max
You have Attention Aware enabled on your phone which recognizes you are looking at your phone after it starts ringing and it turns the ringer volume down on purpose. You are after all now aware the phone is ringing, right?
People around will most assuredly appreciate this, but if you prefer to turn the feature off, go to Settings > Accessibility > Face ID and Attention and turn off Attention Aware Features.