The integration of phone calling into Gmail was supposed to be a US-only feature, but it turns out that anyone who sets their language to US English in Gmail can also use the service to make calls (including free calls to US numbers). However, be careful when using Gmail for calling or you might accidentally switch it off.
Google Operating System points out that you shouldn't click the Upgrade your account button within Gmail if you enable this option, since that feature is there to assign you a Google Voice number, and that service is US-only. Clicking the upgrade button can result in your no longer being able to buy credit to make non-US calls, which makes the service potentially a lot less useful for Australians.
Don't Click this Button in Google Voice! [Google Operating System]