Background: I bought 2 Galaxy S3's. One had the problem out of the box, and the other was just fine. A friend had a problem as well. It seems some Galaxy S3's may have shipped with a defective APN config setting. Method 1 fix is the "real" fix, but it requires a rooted phone. Method 2 is more of a work around and doesn't require a rooted phone. You must use Go SMS Pro instead of the stock messaging app for SMS/MMS messages.
Method 1 - (the real fix, but requires a rooted phone):
***UPDATE***
I've confirmed that this method works on ICS (4.0.4) and Jellybean (4.1.1).
1) Download this working Sprint APN xml file:
http://www.mediafire.com/?jx41lw4w5107frt
2) Download "APN Backup & Restore" from the Google Play Store.
3) Convert the "APN Backup & Restore" into a system app by either using Titanium Backup, or simply use Root Explorer/ES File Explorer (root enable it) to move the APK from /data/app to /system/app.
4) Reboot the phone.
5) Launch the app, click on menu, then click on "Disable ICS Check".
6) In the main app menu, click on "Backup APNs". This will create a directory called "ApnBackupRestore" in your /mnt/sdcard.
7) Copy the xml file from step 1 to the /mnt/sdcard/ApnBackupRestore folder.
8) Go back to the main app menu, click on "Delete APNs" and confirm it.
9) Go back to the main app menu, click on "Restore APNs".
10) Choose the file you downloaded from step 1 and confirm it.
11) Reboot your phone.
12) Done.
Method 2 - (work around, doesn't require rooted phone)
1) Download and install Go SMS Pro to your phone from the Google Play Store.
2) Launch Go SMS Pro and skip through the intro message screens.
3) Skip the Go Chat Service offer unless you really want that.
4) Press Menu button on your GS3.
5) Choose Settings.
6) Scroll down the menu and choose Set default SMS program.
7) Press each entry in the list and put a check mark on "Use by default for this action" then choose Go SMS Pro. When both are done, press the back button on your phone.
8) Up on top, press "Advanced"
9) Scroll down the list and press "MMS Settings"
10) Scroll down the list and put a check mark on "Fill APN configuration manually"
11) Press "MMSC".
12) Type in "http://mms.sprintpcs.com" - do not include the double quotes. Press OK button.
13) Press "MMS PROXY".
14) Type in "68.28.31.7" - do not include the double quotes. Press OK button.
15) Press "MMS PORT".
16) Type in "80" - do not include the double quotes. Press OK button.
17) Press back button on phone twice.
18) Send/receive a few SMS/MMS messages to test.
19) Done.
If you'd like to thank me, please click on the "Thanks" button instead of posting a thank you note. Let's save storage and bandwidth.
