It will either be PUK1 or PUK2
How Do I Unlock My Sim Card?
You need to call up customer service from which ever company your with like Optus, Vodafone or Telstra. They'll tell you what to do. But I have to warn you, you might be waiting on the phone for a bit so grab a good book or something.
Call your customer care service for help :)
Now phone says enter puk code
If you have att/cingular, log into your account and go to support center and type in puk code, it will give the directions to see puk code online, or you can call customer service. I'm sure it works the same with the other carriers