[Sony Xperia T, TL, TX, V] bsined:
This guide is for fellow at&t customers who will be stuck with JB 4.1.2 forever because there won't be an official 4.3 release (and any new Sony phones until at least the next year).

WARNING: You may lose some of your contacts if you don't follow this guide!

DISCLAIMER: I'm not responsible for bricked devices and data loss from power surges, hardware damage, or changes to the linked software. Always check the native manuals for all the tools listed here!

What You Will Lose:

All at&t branding

at&t address book (see below how to save contacts)

at&t emergency numbers dialer

Stable LTE connection (you'll see the H+ icon often in places with weak signal)

NFC icon

What You Will Gain:

Clean ROM with all stock Sony apps

All the new JB 4.3 features

Sony's new camera app with plugins

Significant improvement in battery life if your phone barely survives a day

What You Will Keep:

All of your contacts and data

James Bond themes

Root access

Recovery

Required Downloads:

1. Flashtool (got questions about it? - ask them here) - requires installation

2. [MENTION=4904057][NUT][/MENTION]'s dual XZDualRecovery. I'm linking to v2.7.94 BETA installer. If you wish to check for a more recent version, go to his download page and do a page search (CTRL+F) for XT-lockeddualrecovery[version-goes-here].installer.zip. (got questions about it? - ask them here) - unzip to a location you'll remember (say, C:\xperia-upgrade\recovery\)

3. [MENTION=402300]amarullz[/MENTION]'s AROMA File Manager - current version can be found at the bottom of the 2nd post - don't do anything to the zip

4. [MENTION=4872623]DaRk-L0rD[/MENTION]'s Ultimate PureXT ROM and all patches and/or add-ons from the 2nd post you'd like (got questions about it? - ask them here) - don't do anything to the zip, save it to a location you'll remember (say, C:\xperia-upgrade\ROM\)

4.1 There might be additional files needed for the ROM, such as SuperSU update. They will be mentioned in the post. Download them to the same location as the ROM (e.g., C:\xperia-upgrade\ROM\).​

5. at&t baseband FTF (attached) - unzip it to the default Flashtool firmware directory (C:\flashtool\firmwares\)

​

The Guide (assuming your phone is running at&t's 4.1.2 (9.1.F.1.120):

1. Backup any contacts saved in at&t address book or make sure there are none.

1.1 Open the stock Contacts app.

1.2 Tap the filter icon on the left.

1.3 Make sure that the two checkboxes are blank.

1.4 Go through every single connected account EXCEPT for AT&T Address Book and uncheck them (there might be a few), then tap OK.

1.5 Tap the menu button, then Back up contacts.

1.6 Select SD card.

1.7 You don't need to remember the location of the vcf file - the system will recognize it on its own.

2. If USB debugging isn't enabled yet, you need to do it.

2.1 Go to (System) Settings -> Developer options and flip the switch in the top-right corner of the screen.

2.2 Look a bit down (toward the middle of the screen) and check USB debugging.

3. Connect your phone to the computer and let the computer recognize it.

4. Copy AROMA Filemanager to the SD card

4.1 Go to the location on your computer where you saved aromafm.zip and copy the file (select, then CTRL+C).

4.2 Navigate to your SD card (not internal storage) and create a folder called clockworkmod.

4.3 Open the new folder and create another folder inside it called aromafm.

4.4 Open that folder and paste the zip (CTRL+V).​

5. Copy the ROM and all of the files required for it to the SD card.

5.1 Go to the location on your computer where you saved the ROM zip with all other required files and copy the files (select, then CTRL+C).

5.2 Navigate to your SD card (not internal storage) and create a folder called 00-install (such a name keeps it on the top of the content list in Linux).

5.4 Open that folder and paste the zips (CTRL+V).
​

6. Install the dual recovery.

6.1 Go to the location on your computer where you extracted dual recovery (e.g., C:\xperia-upgrade\recovery\).

6.2 Double-click install.bat.

6.3 Press 3 on your keyboard for the option 3. Installation on unrooted ROM.

6.4 Follow the instructions on your computer's screen.

6.5 When you will try to reboot the phone back into Android, the recovery will prompt you to root the OS. Do that.​

7. Backup old system and install the new ROM (YOU MUST DO THIS!!!) - if you skip this and have issues, later, I'm not responsible

7.1 Initiate the reboot sequence by holding POWER + VOLUME UP for a few seconds.

7.2 Once you see SONY logo, start pressing VOLUME UP to enter clockworkmod recovery.

7.3 Connect the phone to a power outlet.

7.4 Tap Backup and Restore

7.5 Tap Backup to /storage/sdcard1 and confirm. The process can take up to 10-15 minutes depending on the amount of data in your internal storage.

7.6 When backup is complete, tap the Back button (2nd from the right) to go to the main menu.

7.7 Tap Wipe Cache Partition (4th from the top) and confirm the operation.

7.8 Tap Back button (2nd from the right) to go to the main menu.

7.9 Tap Advanced (3rd from the bottom), then Wipe Dalvik Cache (1st from the top) and confirm the operation.

7.10 Tap Back button to go to the main menu.

7.11 Tap Install Zip (2nd from the top), then Choose zip from /storage/sdcard1, then select the folder 00-install, then the ROM file, and confirm the operation. If you receive Status 7 error message, leave the phone as is, scroll down to the troubleshooting section to restore the backup and fix it.

7.12 Repeat step 7.11 for all the files that need to be installed.

7.13 When you are done, select Reboot System Now in the main menu. It may take good 20 minutes for the system to perform all operations needed for the first start.​

8. If the phone doesn't pick up LTE signal, you need to re-flash the at&t baseband.

8.1 Turn off the phone.

8.2 Start Flashtool from your Start menu.

8.3 Click the flash icon, select Flashmode (default), and click OK.

8.4 In the list of firmware select the one you've downloaded (if this is your first time flashing, it'll be the only one for LT30) and make sure that No final verification is checked. Then click Flash.

8.5 When prompted, hold the VOLUME DOWN button, and connect the phone to the computer. The operation will take about 10-15 seconds. DISREGARD THE INSTRUCTIONS IN THE ANIMATION. Xperia T/TL doesn't have a hardware Back button.

8.6 Boot the phone.

8.7 Sometimes the phone will still lose LTE signal and show H+. You can reconnect to LTE by putting the phone into Airplane mode for a couple of seconds and then turning the Airplane mode off. If that doesn't help, open dialer, type *#*#4636#*#* to get into Testing service menu. There, you need to tap Phone information and scroll all the way to the bottom where you'll see a selector with the label Set preferred network type. Pick the flavor of LTE and downgrade that you'd like and watch the connection be re-established. Unfortunately, through experimentation, I've discovered that every ROM keeps its regional baseband regardless of flashing method (even overwriting with NAM bands will only enable LTE reception but the baseband property stays the same).

9. Battery fix if your battery dies within 9-10 hours of use.

Apparently, in some areas at&t doesn't support a network feature called fast dormancy (it allows for somewhat faster reconnection to the data network after idling). Some apps aren't written properly and keep on pinging Sony's FD service to connect, which results in thousands of wakeup alarms and the phone never entering deep sleep. You can notice this if Android OS is your top battery hog in standard battery usage stats (com.sonyericsson.android.wakeup is the service name). Before applying this fix, check your battery drain using BetterBatteryStats or Wakelock Detector. If you see what I've described, this will help you increase your battery life by at least a couple of hours (for me, it almost doubled).

There's one downside though: the last 10-15% of battery charging take much longer (my battery now charges over 3+ hours).

Huge thanks to [MENTION=4900593]mirhl[/MENTION] for writing the original guide.

1. Install some root-capable file browser (I prefer ES File Explorer for being free).

2. Grant it root access (in ESFE, tap the top-left corner to open the navigation sidebar, scroll down to Tools, tap it to expand, scroll down to Root Explorer, and tap the switch).

3. Mount /system as R/W (in ESFE, get to Root Explorer as above, but instead of the switch tap the tile itself, then tap Mount R/W, and move the radio button next to /system into R/W column).

4. Navigate and open /etc/fast-dormancy/fd_custm_conf.txt (ESFE has a built-in text editor but editing needs to be enabled through the top-right menu.

5. In the file, you'll see two lines ending with MCC and MNC and a zero next to each. Change the zeros to 310 (MCC) and 410 (MNC), save the file, and reboot the phone. That should take care of it.

IF SOMETHING WENT WRONG

1. There shouldn't be any issues until you complete 7.6 (phone backup).

2. You better have followed this guide and created a complete backup of the working setup.

Bricked phones (something went wrong and you turned off your phone and now it doesn't boot)

1. Download a 4.1.2 FTF file from this master list to C:\flashtool\firmwares\.

2. Remove the SD card from the phone just in case.

3. Follow the instructions in the master list on how to flash the firmware.

4. Insert the SD card back into the phone.

5. Repeat steps 2, 3, and 6 of the guide above to install recovery.

6. Boot into clockworkmod (7.3 above), navigate to Backup and Restore, then Restore from /storage/sdcard1 and select the appropriate backup. This will get you up to step 7.6 and you can try the guide again.

Status 7 error message when flashing a ROM

After some experimentation, I noticed that some rooting methods are the culprit. You need to unroot your phone (don't worry recovery installer will work either way and will reroot it safely). Just download this Xperia rooting script, connect your phone with Debugging access enabled, run the script, and select the appropriate option (unroot). When it's done and the phone reboots, reinstall recovery (it will prompt you at one point to root the device), and continue to 7.7.

Huge thanks to all the devs who have built the tools!​

Changelog

v2 - Apr 1, 2014 - Added extensive troubleshooting & battery fix; formatting changes
v1 - Mar 26, 2014 - Original publish