[Galaxy S III Q&A, Help & Troubleshooting] Solidcancer07:
Hey all again, if anyone is reading this that is... :p

Surprisingly enough, I flashed the stock 4.1.2 United Kingdom BTU firmware, and everything is working as normal. O_o

I would've expected there to be "Not registered to network issues", but texting, calling and mobile internet are all working fine.

Note that I didn't even flash the patched modem as mentioned in the guide yet, the phone worked as normal after flashing the stock 4.1.2 UK BTU firmware and even recognised that I was roaming (currently in the Netherlands!! :D). Don't ask me why, but it just works... which is AWESOME because I have a functioning S3 again without any further patching/modding!!!

For those of you wondering, my IMEI also comes up as normal, and again no "Not registered to netwrok" issues.

For anyone wanting to try and my results, here is what I did:

FIRST AND FOREMOST BACKUP DATA. THIS IS A DANGEROUS OPERATION AND COULD POTENTIALLY BRICK YOUR DEVICE. PROCEED AT YOUR OWN RISK. I AM NOT RESPONSIBLE FOR ANYTHING ON THIS THREAD AND FOR ANYTHING THAT WILL HAPPEN TO YOUR DEVICE AS A RESULT OF FOLLOWING THIS GUIDE. BY FOLLOWING IT YOU ACCEPT THIS.

1. First, just read the guide above. Don't do anything, just read it. Here once again: Downgrading from 4.3 to 4.2.1 and the No Network Issue...

2. Now back-up your EFS. I CANNOT STRESS HOW IMPORTANT THIS IS (hopefully bold caps will work...).

You will need root, so follow this guide if you do not already have root: How to Root & Install CWM on Samsung Galaxy S3 [Guide]

I used the following thread to backup EFS: [TOOL]Backup/Restore EFS (IMEI) for all SGSIII . I used the SAMSUNG_I9300_EFS_BACKUP.zip file to backup my EFS. NOTE: If you don't know what EFS is, read [REF] IMEI / EFS - Know this stuff (i9300) - including "no network"

3. Now download the stock firmware you want to downgrade to. In my case it was 4.1.2 United Kingdom BTU from SamMobile. (I9300XXUGMJ9_I9300OXAGMJ9_I9300XXUGMJ9_HOME.tar.md5)

4. Before you install stock firmware, we need to use TrianglAway to reset binary counter to 0: [2013.10.04][ROOT] TriangleAway v3.10 This is because rooting causes binary counter to increase.

Install the .apk as normal and run. Follow all instructions carefully. When TriangleAway says "press volume up to reboot", press volume up, and then when the screen goes black quickly pull out the battery. After 10 secs put the battery back in. Hold down Vol Down + Home + Power. Then press volume up to go to Download mode. You see binary count 0 :)

5. Pull out the battery again. Wait 10 secs. Put battery back in. Hold down Vol Up + Home + Power to enter stock recovery. Scroll down to Wipe Data/Factory Reset and press Power to select. This will wipe your phone clean, removing 4.3 update and data.

6. Pull out battery when done. Wait 10 secs. Insert battery again. Enter download mode again by pressing Vol Down + Home + Power. Then press Vol Up to enter Download Mode.

7. Open Odin, and connect your phone. Select PDA and select the .tar.md5 official 4.1.2 firmware you want to downgrade to.

8. Wait for Odin (will seem like Odin has froze but it is just checking md5 of firmware). Then click Start.

9. Once Odin is done, it will inform you and your phone will automatically restart into official stock 4.1.2 firmware! Go through setup process and finish. Now check your IMEI. And try to call a friend. And text another friend to see how they are. At this point my S3 was working normal, just like it was before the 4.3 update.

There you go, these are the steps I followed to downgrade my phone, without applying patched XXEMG4 modem :D hope it works out well for you guys as it did for me :)

If this doesn't work, then flash the XXEMG4 patched modem as stated in the guide above or in step 1. I have not tested so am unsure of results.

Have a nice day ;)