Hi, Yes you can buy a prepaid mobile phone card with credit and when it runs out you can top it up. Your phone must be unlocked for it to be able to work. To check this try someone else's sim card in it from a different company to yours.
I found that the Simpati network worked trouble free on my phone. (I've tried XL, that sucked.) The 50,000 rupiah charge card gives you around 30 minutes talk time back to Oz.
My mate used the phone at the airport to tell his girl of a flight delay to Perth. They require a credit card to work. On his next statement he was debited nearly Aud$60 for the 3 minute call!