I know that in Australia you are using different kinds of phone systems, so I would like to draw your attention to the fact that Thailand is using the GMS system, and make sure your phone is an unlocked GMS phone.

The most convenient thing is to buy a new local SIM card in Thailand (1-2-Call, or DTAC are the two biggest companies and gives you the best coverage). Price would be about 350 Baht including some 50 Baht free usage. Phonecards with additional time airtime can be bought almost anywhere for 100, 150, 300 or 500 Baht.

Send your family and relatives a SMS with your new local phone number - and you are connected to the whole world. There are discounts for overseas calls by dialing prefix 007, 008, or 009 before the country code depending of which local phone company you are using. But check that out when you get there.