But 'absolutely quiet' is very non-scientific, isn't it, as it depends on lots of factors like the quality of your phone, whether you're finding it difficult to hear, whether you're in a room that is near a busy road, whether your neighbour has chosen to hoover the rooms and all that.
People just repeat stuff about the quiet line test without actually critically engaging in how accurate such statements are. If you want to test the quietness of the line, you should do it in a lab and in controlled conditions and using appropriate equipment.
Other than that, you may just have a vague idea that the line is quiet.