We don't get Google Voice here in Australia, but we've got a better option - Aussie developer BigTinCan has an app for pretty much every mobile platform that lets you completely manage your phone connections, from having local landline numbers all around the world to sending calls from your ex-wife straight to a very "special" voicemail.
The app allows you to make and receive phone calls at a cheaper rate than your mobile phone. When you want to make a call using the app, the software sends a small data packet to the BigTinCan servers, which verifies your account, and then calls both you and the number you dialled. That means that you don't pay your mobile carrier for the call - because BigTinCan calls you, you don't pay that rate.
Rates are competitive, and the calls aren't through VoIP, they're through standard phone lines - BigTinCan resells a wholesale service through a Telco partner.
But the cooler elements of the service are the ability to customise your numbers. You can purchase a local number for places around the world for a certain period of time, and not only have people who call that number redirected to your mobile automatically, but you can also dictate specific actions depending on the caller ID, the time of day or any number of other features.
Like if you buy a UK number, you can set it up that if somebody calls that number outside of the 9-5 working day in Sydney, it automatically redirects to a dedicated voicemail. If they call between, let's say 12-1pm, they get a different voicemail.
Everything's paid for over Paypal, and you buy credit in advance.
It's a pretty cool app from an Aussie developer, so if you need to make a lot of calls around teh world and only want to receive calls on your mobile, make sure you check it out.