A number of phone plans now include some international calling credit, but the amount of calls you can make, and to which countries, varies a lot from carrier to carrier.
Telstra, Optus, Vodafone and Virgin Mobile all include some international calling credit in prepaid, SIM only and Phone Plans - typically on plans costing $50 or more.
Lebara Mobile is an MVNO which is dedicated to cheap international calling rates. Many of its plans include some credit, and one plan includes unlimited international calls to 60 different countries.
Note: be sure to read the fine print. Some plans offer calls to landlines only, while others allow you to call mobiles in the destination countries.