Maybe the filtering happens at low level, like “firmware”/drivers of the VoLTE signal, that part is handled by Fairphone, so you might have an answer from them if that is the case.
No, I saw your thread but I don’t think it’s quite the same.
My audio is consistent and otherwise fine, the noise reduction just seems a bit overzealous (it filters out cars and footsteps as well to the point that people can’t tell I’m outside, which is nice at least). I’m not from Germany either, although my carrier hasn’t been able to help.