I have a first generation iPod Touch so I'm not sure if this would work on it, considering it has no external speaker. Even with headphones with a mic, I feel I wouldn't be able to install the app on 3.-whatever the fuck my iPod is limited to.
For my Droid and calling home, I use GrooveIP. Works just the same way with your GVoice number. Although, you can only call Canada and the US. So it's basically the GMail function on your desktop, but for your phone. The GVoice app for Android, I don't think, let's you call for free just on its own. GrooveIP let's you do that.