I phone 11
My incoming ring volume is decreasing as it rings ,one loud ring,down to too low ring,what’s up
iPhone 11, iOS 14
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It is designed to automatically decrease the ring volume as soon as it recognizes that you have heard it. If you don't want that, turn off Settings > Face ID & Passcode > Attention Aware Features.
See: About Attention Aware features on your iPhone X or iPad Pro - Apple Support