In Reply to: Optus posted by screamin on Wednesday, 29. September 2010 at 05:18 Bali Time:
If you wish to use your sim from home in Bali, no matter what network you're with, you'll need to enable international roaming before you leave home. Be aware that it's very expensive to use, and much cheaper to buy a local prepaid sim to use while you're there - so long as your phone isn't locked to your network at home.
If you mean that your sim didn't work to call from Australia to Bali, then it will depend what plan you're on, and if they have barred international calls for you.