Very odd, but I dont see a way to do this either. We have AWC, and I just setup a conference, and called in with leader code from one phone, and participant from another. Neither could access any menus (hitting all keys on keypad), so I'm guessing there is no way to do it. Sure, from a web page, I can mute people, see whos talking, disconnect people, etc.. but from the phone; nothing.
It amazes me that the simplest things are left out of AWC. Of course I'm not expert on AWC, so maybe theres something somewhere to enable phone commands, but so far, I have not found it from reading docs, or looking around all the server admin screens.