my phone ring sound decends in volume when ringing

You have Attention Aware features turned on your iPhone. This is by design and is actually a really thoughful feature. Once the phone knows you are looking at it, the volume of the ringer is lowered, because well, you are aware your phone is ringing. In situations where there are people around you, they will most assuredly appreciate this. If however, you don't want your phone to do this, you can turn off Attention Aware features, by going to Settings > Accessibility > Face ID & Attention > turn off Attention Aware Features.

You're welcome