It didn't happened to me. A friend bought a wile ago a Samsung galaxy s3 network free from e-bay. After the jelly bean update, something happened. The phone was not sim free any more. After searching up and down on websites and forums, I found like 2 veritable solution.
One was 100% free but takes a little more time(but free) and the other one by downloading a free app from Google play and after installing it to pay around 12$ to make the job for you. If you don't want to do it yourself for free just write down free sim s3 on market And you will find it easy.
Today here we'll talk about the 100% free way to sim unlock on galaxy s3.
You need to check if your phone it's Samsung Galaxy S3 i9300 because it will work only for this version.
You need to Root your phone if is not rooted. Do not worry if you don't want it rooted, after you unlock your sim you will un-root it back.
Here you can find an easy way for how to root your Samsung galaxy S3 HERE! and go to root section. But read carefully and fallow the instructions there with attention. If you can not handle it ask me here on comments area and I will help you. For example for my friend's Galaxy s3 I used this method HERE!
After your phone is rooted come back for next steps here.
Next step, find on Google play for free and download and install Busy Box.
Next step download this
After you download it, put it on your phone memory and install it. You can find it after you lace it on phone won My Files on phone where you copied it on phone. After installation, open te program and you will see something close to this image:
At this point the program will ask you for busy box if you don't have it installed on phone. If you do, just press on all ALLOW and Unlock!
If you see all lines green it means you've done it. Congratulation!
If you need help with something, just ask me here and I'll be glad to help you.
Custom [ROM] ROOT