Should be easy to use.
Such as (im chosing binding randomly): Push to talk, hold T to talk to team, hold G to talk global.
When you hit tab, and bring up that ingame menu, have a mute button next to people names.
Fix the GUI to have ally pics with hp/mp bar (no specific numbers, just a basic bar). Highlight the picture,and add a speaker on it when that person is talking. For global, just put their name up top, glowing with a little speaker.
Personaly I think a chat system like BF2142has would be great. It has a great built in voip, and the commo rose chat is grea too (hold a button and default chat message show up on screen. Click the one you want).