Adjusting handset sensitivity

I have some desksets in a room with a lot of HVAC white noise. When speaking with folks in that room the white noise is very distracting.

I think I need to adjust the gain on those handsets lower. Should I look to do this as an attribute of the handset or can this be done on the freePBX system? If so could someone point me in the right direction?

Thanks!

handset

Thanks!