Normal exit code is 001, but to get a cheaper rate you should dial 007, 008, or 009 (exit code) + 61 (for OZ) + area code + phone number.

You may also buy a TOT card (Telecommunications of Thailand) in any 7/11 supermarket for 100 or 300 Baht and use the phone for international calls just outside the shop dialing 007 for cheaper rate.