Mobile data is, for most people, cheap. It's cheaper than most mobile plans' included call credit, especially if you're calling someone in another country, and especially if you're on free Wi-Fi. If you have an Android phone, the international might of Google is making it easier for you to place telephone calls, both locally and internationally, without falling prey to your mobile carrier's convoluted rates.
Hangouts got a desktop app update recently that gave Chrome browser users a desktop-esque, Facebook chat heads experience for their online conversations. That's great for text chatting with your friends when you're both sitting at a PC, but for mobile users on Android, Hangouts just got a big bump for voice calls, bringing it into competition with Skype and Viber and all the other data-to-voice carriers out there.
This newest update to the Hangouts app on Android — it's Hangouts version 2.3, and the Hangouts Dialer app that goes along with it — lets you use Hangouts to make voice calls to any phone in the world, even though in Australia we still don't have access to Google Voice and the entirely-online mobile phone numbers that come with it.
There's no restriction on the numbers you can call using mobile data — within Australia, for example, it'll cost you US$0.02 per minute to call a landline and US$0.07 per minute to talk to someone on a mobile. Funds won't be drawn from your Google Play account, so you'll need to add credit specifically for use in Hangouts; this is a bit of a pain if you use Google Play gift cards to save on the cost of apps.
So, if you install the Hangouts update and the Hangouts Dialer that goes with it, you can make very cheap voice calls to any mobile or landline in Australia or around the world. You can't receive mobile calls with it — since you won't have a Google Voice number — but if you make a lot of international calls regularly, and have mobile or Wi-Fi data to spare, it might turn out to be worthwhile. [Google]