Volume on a extension is set by what the phone is doing, ie to make a phones ring volume louder or softer requires the phone to be ringing while adjusting the volume buttons.
To make a phone ring you need to look at where the lines are ringing in mmc 406 then assign extensions to that group. Example lines ring station group 501 then add extension 201 to the 501 group.The group can be set up different ways but you probably would set ring type to unconditional so all phones in the group ring at the same time. The next destination would be where the unanswered calls are sent to voicemail. At the least google yourself a manual if you don't have one, without it you are going to waste a lot of time.