Individual user volumes can only really be adjusted if the user is an IP phone. If they are, then under the EXTENSIONS section, pull up that particular phone's settings and go to the second tab. You'll be able to adjust GAIN values. Be careful, as if the site has Analog lines, you can get annoying echo if you boost it too much.
Do they use a headset? If so, its possible the headset settings need to be adjusted/tweaked on the headset's interface box.
Hope this helps...
Kris G.