The theory is that the S3 has, in the boot process, a routine which is "tripped" when it detects an error condition. This routine is designed to "save" your IMEI by restoring images from two built-in backup blocks. The problem is, it actually does the opposite; for whatever boneheaded reason, the default backup blocks contain dummy images with an IMEI of 0 and no carrier identification. This is why your phone forgets what carrier it's on and what its IMEI number is; the restore routine actually overwrites the phone's only copy of the IMEI data with a dummy. Because much more data is lost than just the IMEI, this is also why a QPST injection (not a QCN restore) persistently leaves you on roaming.
In an initial attempt to solve this problem, users have attempted to "dd", or create an image of, a single block supposedly containing the IMEI when the phone is still in a good state. This is the efs.sbn method that you may have seen. However, upon further inspection, it turns out the IMEI data is actually spread across five different block devices, hence why the efs.sbn method is either ineffective or leaves you on roaming! After the block devices were identified, a SynergyROM team member created an easy-to-use recovery zip that allows you to image these five blocks, allowing you to reflash them should the Samsung boot process overwrite your IMEI! Re-flashing these blocks after they've been crapped up virtually ensures your phone has been fully restored to its pre-IMEI-loss condition, thereby preventing roaming or data issues seen consistently with a manual injection, or intermittenly with a QPST restore. You do not need SynergyROM to use this tool; I've done it from full stock.
But wait: there's more. About a week after the development of this tool, several unused boot entry points were discovered that revealed a way to control Samsung's built-in "safety" feature and actually make it work properly! Two backup blocks were put in by Samsung themselves; "fsg" and "backup". The great news is that, although these images may contain dummies by default, you can instruct the bootloader to write your good NVRAM data to them so that the good data is what's written to NVRAM in the event of an error. That is, instead of restoring a dummy image over your good IMEI...it restores a good backup instead and you don't lose a thing! How awesome is that?
Developers are now recommending a new IMEI backup regimen, leveraging Samsung's built-in IMEI backup and SynergyROM's manual backup, together with an optional QCN backup you can do for good measure. Of course I can't make any guarantees, but doing this backup should leave you in such a good spot that almost nothing can total your IMEI or leave you in a roaming state.
Question Authentication error while connecting to your email provider
Facebook
Twitter
Instagram