Sometimes no sound for incoming calls

Hey! I got iPhone 12 Pro, and something weird is happening. I have noticed that sometimes I am not alarmed about my incoming calls. Basically my phone is next to me, sounds are on, but I don't get any notice on incoming call - I just notice a "missed call" notification when I check my phone later. And this is "selective", for example, today I got 2 calls - one with sound, and the other was missed because no sound. The same happens with messages from different apps - the sound is "selective".

I have not muted my phone, the sounds are on.