People who like to flash their smartphones mostly end up corrupting their IMEI and radio that actually almost ruins your smartphone. Although this problem is not pretty common, thankfully, but that don’t mean that it doesn’t exist. Though there is now not one but two methods available to fix it for AT&T Samsung Galaxy S III and for other US variants as well. There is a tool available that will restore your IMEI without any haste and a manual guide for those who likes to get involved and do things by themselves.
Both methods have got the same objective that is to back up and restore the IMEI, so chose whatever you like but there are some pros and cons of both methods. Manual one is arguably the sound one and supports all the US variants, though this method hasn’t yet been tested on Sprint Galaxy S III. While the tool provides a much shorter and easier root free method. So if you have a corrupted IMEI then you can fix it easily with the help of these two methods, and the good thing is both the methods have tutorials for you that will make things further easier. So check out this thread if you would like to try the manual guide or else check out this one if you want to try out the tool that possibly will restore your IMEI.
Source: XDA Developers