Check the original carrier of Galaxy S3 How do I check the original carrier for Galaxy S3 (model no is i9300 in Australia)?

I bought it from one buyer. I can't use the network. So, please help me.
i9300 is international model of Samsung Galaxy S3. So it won't come up with any specific carrier. You can also use i9300 with any carrier as you like.
but there are some other models with specific carriers. If you want use them ,you need to unlock it.
Like:
1747- At&amp;T US....

You can check carriers and S3 model names here