Been rooted incorrectly and its wiped your IMEI. Usually Samsung's that get flashed saves the IMEI in a special EFS partition. Only accessible by special programs.

If you recentky bought this S3, its been messed with and if you have no knowledge of Rooting, there is not much you can do... Except asking the right questions on XDA-Dev.

I had the same with a S3 ages ago, but DID manage to solve it because I used the correct program from the start.

If its an eBay or Facebook purchase, get a refund.

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