Why does the ringer volume change on iPhone XS Max?

Question

iPhone

hello
, there is an iPhone XS Max 256GB PCT, bought new in the store
, the latest iOS is installed .
Approximately in the last month, incomprehensible things began with the volume of the incoming call.
It manifests itself as follows - when an incoming call comes in, the sound of the melody plays at maximum volume! This does not happen all the time, but approximately 50/50, i.e. 1 time is normal, another call is loud.
The sound in the settings is set to about 30% of the maximum, I tried to change the sound in the settings, rebooted the iPhone - nothing happens.
I climbed all the settings - I did not find anything wrong.
The phone did not receive any damage.
Where else can you go to see? I can not understand at all what is the reason for such "glitches".
I know about the definition of "attention" in face id. Glitches happen when I'm not looking at my phone.