iphone 11: ringtone is quiets down after call

That sounds like the option that is designed to lower the volume automatically as soon as the phone recognizes that you have noticed the ringing and started to respond to it. That option is at Settings > Accessibility > Face ID & Attention > Attention Aware Features.

Note that if that is what it is, after it has lowered the volume for one call, the volume will be high again when the next incoming call starts to ring.

FoxFifth is correct. You have Attention Aware enabled on your phone and it’s doing what it’s supposed to do. It’s actually a great feature as once your phone recognizes you are looking at it, the volume of an incoming call is reduced. People around you will greatly appreciate this feature.

You’re welcome.