For years now:
I have been opening the Plantronics headset boxes of people that have annoyingly loud phone voices.
I have been surreptitiously turning the input volume up as far as it will go.
As it seems nobody knows the input volume control is there except for me, this has a positive effect: on the Loudy's next phone call, the
victim recipient generally asks why they're shouting, and from then on, a markedly more measured tone is gradually trained into their phone conversations.
'Course, it doesn't work at all if they don't have an amplified headset box thingy.
I don't know why I'm revealing this now, but I know why I thought of it.
Hopefully your workplace can be made better (read: quieter) too, through guerilla volume adjustment.
Global Louding is real. It's happening all around us. We must fight back.
 There's another pull-off panel next to the battery cover, and in there, there's a tiny flat screw that adjusts the mic volume (and a convenient little plastic tool you can use to adjust it).
 I personally contribute to noise pollution by playing music after 5. I have eclectic tastes.
 Open plan workplaces are the worst for this sort of thing - whatever (expletive deleted) decided callcentres - actually, forget callcentres, *anywhere* would be better when everybody doing the talking has direct line-of-sight with everybody else was possibly the most inspired evil idiot-savant genius in modern history.