[T-Mobile Samsung Galaxy S 4] Frankenscript:
Hi everyone,

A month or so ago I got sick of the locked bootloader of the ATT I337 variant of the Samsung Galaxy S4. The early firmware versions on this phone (before MK3) had boot loaders that while locked, could be worked around with known exploits, allowing proper recovery installations like TWRP and CWM, and full ROM installations and custom kernels. Alas, starting with MK3, the bootloaders were locked up tightly. The only recovery possible was SafeStrap, which only supports the use of Touchwiz ROMs. My phone was NC1, so for me it was only Safestrap-compatible ROMs.

I had heard that some people with ATT service used the M919, but I couldn't find a decent thread that talked about the specifics, to give me assurance that everything would work OK. But it seemed like a good bet, so I bought one off Swappa. The phone has worked great for me with no problems or limitations whatsoever. I decided to post a description of my experiences to help others that may be interested in doing like I did.

Using the phone, stock, on ATT:

My phone came SIM unlocked, and from what I've read many of them are this way. All I had to do was insert my SIM card, boot the phone, and it found the network. However, to get it to work, I had to replace the T-Mobile APN information with suitable ATT APN. The settings are here, but for convenience the settings are as follows:

1. Go to Settings > Wireless and Networks > More Settings > Mobile Networks > Access Point Names

2. Click the "+" at the bottom

3. Enter settings from one of the configurations below

4. Click "menu" button and "SAVE"

LTE Configuration

Name: ATT Phone

APN: phone

Proxy: Not set

Port: Not set

Username: Not set

Password: Not set

Server: Not set

MMSC: http://mmsc.mobile.att.net

MMS proxy: proxy.mobile.att.net

MMS Port: 80

MCC: 310

MNC: 410

Authentication type: None

APN Type: default,mms,supl,hipri

APN protocol: IPv4

APN Roaming protocol: IPv4

Turn APN on/off: APN turned on

Bearer: Unspecified

Mobile Virtual Network Operator Type: None

Mobile Virtual Network Operator Value: Not set

Note that the last two settings were not visible on my phone on 4.2.2, so just ignore them.

You may also need to go into the Prefered Network Type and set it to LTE... mine stuck on 4G until I remembered to do this step. After this, it worked fine on LTE networks.

If yours comes without being SIM-unlocked, this thread is the best resource I've found for methods to do the unlock procedure. There's also this thread.

Rooting & Custom ROMs:

My M919 came from a vendor that sells a lot of refurbs from T-Mobile. I lucked out and got one that has the MDL bootloader, which is fully unlocked and knox-free. If I had gotten a later one, such as the NB4 that is probably most common now, it would have had the unlocked boot loader but would have the knox counter. There's no known way to remove knox, so flashing a custom boot loader on MK2 and later versions of the M919 will irreversibly trip the knox counter, which is generally reported to void any warranty on the phone. See this great thread on knox, if interested.

Anyway, my goal was to install custom ROMs, so after I was satisfied the phone was working well on the ATT network, I flashed TWRP recovery to it via ODIN. There was no need to pre-root the phone to do this. I just put it into download mode and flashed TWRP. I used 2.7.1.0 although there are newer versions (2.8.0.1 is latest as of this writing).

I had no need of a backup, as I hadn't done much setup on the phone, and I could always flash the original firmware from the Mega links.

I used the Dirty Unicorns ROM, and flashed it according to the Dev's instructions.

I eventually updated to the NB4 GSM modem (the modem.bin only), but was careful not to mess with the LTE modem that is paired with the bootloader (wifi/sound problems happen if you flash an unmatched LTE modem). Here's the thread that describes how. I didn't notice any particular improvement in the use of the phone, but the phone works pretty well in most places I find myself.

Here is a thread I did comparing signal strength bars from phones which may be of interest if you feel your custom ROM "isn't getting good signal." It's mostly that AOSP reports lower bars for any given signal strength versus Touchwiz (or iPhone for that matter).

Since I'm using a ROM that is already rooted, I didn't have to perform a root procedure on my phone, but here is a thread discussing rooting 4.4.2, and an earlier thread with a wealth of information about rooting and such.

I hope this collection of links and experience information is helpful to someone out there. This community has been such a great resource to me in my use of Android devices, I wanted to give back a little, even if it is just putting some experiences together with handy links.

So for any of you out there with ATT devices, thinking of coming over to T-Mobile to get unlocked boot loaders, I can confirm my experience was a pleasant one, and all is working well on ATT with the "T-Mobile" M919 S4.