OK, I think I know what happens. In gmail, first I went to "try the new hangouts". Then at the left panel under my name, indeed "revert to old chat" appeared at the bottom. So I reverted it back to old chat, and this is where I'm now. In both cases, I went back to google voice settings and still cannot see how I can add gtalk. The only option is to "add a new phone" regardless whether it is a new or old chat.