Are you currently in Australia now or still in the US? Yes, you can call from Australia to the US. What you first need to do once you purchase one from magicjack.com or from a radio shack store is to set it up in your computer. Obtain a local phone number of where you are living in the US. Once you do that, any calls to the US and I believe in Canada are free. Please check about the Canada part since sometimes terms might change.
Magicjack comes into two different types. One is the USB connection that plugs directly to your computer. Then you just need to plug in the phone jack.. as if you were doing it on a phone land line. But it will go on the magicjack stick. Then you need a phone to be able to talk. The new magicjack Plus is the one that has one end connected to your router. If you do this and keep it on, you don't necessarily have to have your computer on all the time. The magicjack Plus will be a little more expensive. Check the website or radioshack for prices.
You just pay for the yearly fee. Then have the option to get packaged years for cheaper price. Or just renew year by year. I would recommend that your internet access is at fast enough because it would determine the quality of the phone conversation.
Once you have set up your account and have a computer and phone, you can be anywhere in the world and be able to call the US or Canada for free. However, there might be an option that you maybe able to call other countries for a fee? We've mainly used it to call the US while we were abroad on vacation,etc.