please help me i9300 not network

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http://forum.xda-developers.com – good day I have my device samsung s3 galaxy. Instead of the original imei erased and written in a different IMEI. I want to install a different rom or custom rom now I want to install the network going. we have to throw again imei with spt box. The most robust way to back up efs file? original or subsequently thrown can make the device can erase imei? (HowTos)