Identify Unlocked Device's Carrier I bought an unlocked Samsung Galaxy S3 I9300 a while ago, this phone was imported by an independent store and not any of the local cellular carriers.

And my phone is not getting firmware updates, I suspect the carrier is not releasing the updates but I can't check it because I don't even know what carrier my phone is set to.

How can I tell what carrier my phone "belongs to"?
Galaxy S3 I9300 is international version.

Look at the S3 variants.
S3 belongs to specific carrier has specific model numbers according to Country and Carrier.
Are you sure about it was an unlocked version??