In Reply to: optus posted by potter on Friday, 15. May 2009 at 21:17 Bali Time:
I've used my phone which is connected on an Optus post-paid account in Bali before I got a local sim card to use. I've only sent a text message, but I can't see why a phone call wouldn't have worked. I just turned on the phone and it automatically found a local carrier to use.
Still, the help center is a 1300 number, so a call to ask them directly would probably be the best idea, just to confirm your pre-paid will work.