You can do exactly what he's asking for a one-time cost of $45 with OBiTALK. I used it on my iPhone while traveling in Europe to make several flawless calls to the US, each lasting an hour or more.
If you are logged into Gmail with voice then Obitalk will not ring when you get a call.
Google Voice will only ring one Google Chat endpoint. You have to sign out of all other Gmail and Google Chat sessions for it to work. It's a nice solution, but the Obitalk site should mention this limitation.
If you use an Obihai device, I highly recommend that you setup a new Google voice account for it, separate from your main Gmail account, for just this reason. You still get an extra phone line that you can make and receive calls on, and you don't need to worry about your Gmail session intercepting your phone calls.
