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To add insult to injury, the higher ranked personel usually have the loudest, most annoying ringtones. Naturally, they have them on full blast during meetings. I'd be surprised if they even know how to use the vibrate function on their phone.
Outside of the office, pretty much everyone has their voice and phone on super loud mode. My guess is that it stems from the same reason: to show stature. The boss in public just acts as if he's in the office, by talking loud as usual. The subordinates talk loud in public perhaps to release their frustration of not being able to do it in the office, or perhaps to show that they are their own boss once the work day ends.
Anyhow, it is a nuisance and another stain on the international image of China.