For those of you who want to unlock SIM on your AT&T or T-Mobile Galaxy S4, there’s an easy way to unlock SIM as outlined by XDA user kaleido.
This should work on AT&T Galaxy S4 SGH-i337, T-Mobile Galaxy S4 SGH-M919, and also all Canadian SGH-i337M variants.
Remember, unlocking SIM is illegal in the U.S. as of this year, and that is why I cannot make a video tutorial but this method does work well, you can find full step-by-step tutorial at XDA here. However, it is not illegal to unlock your SIM if you are currently in another country other than the U.S. This law should change soon as it only benefits carriers not consumers.
The process involves getting into Service Mode on your Galaxy S4 by typing “*#27663368378#” on your dialer and messing with some of the settings similar to the “Note 2 SIM Unlock method.”
To test the your Galaxy S4 has been fully unlocked, you need to insert another carrier SIM on your phone. E.g. If you are using AT&T S4, you can insert a T-Mobile SIM card to test this out.
As a bonus note though, if you are a current customer of AT&T, they will just give you your unlock code if you ask for it and also same goes for T-Mobile. In case they don’t give you unlock code, you can use this method.
Why would you want to unlock SIM?
When you unlock SIM on your Galaxy S4, you are not locked down to AT&T or T-Mobile, you can use your phone worldwide on any network that supports SIM cards. E.g. You can turn your AT&T Galaxy S4 into a full-fledged T-Mobile S4 by unlocking your SIM then applying the AWS band MOD.
What is a Custom ROM?
A custom ROM is a custom, modified operating system that you can install on your Android device. Basically it's like installing Windows OS on a Macbook. By installing a custom ROM, you can bypass stock factory settings like locked hotspot tether, install new Android versions early, or even convert your phone into another OS.
With a custom ROM, you can really maximize the capabilities of your smartphone by adding custom features that have been developed by open source developers.
How to Install Custom ROMs
First, you will need to root your device, if it is not rooted yet, follow our root tutorial and root your phone.
Once rooted, you should have TWRP installed on your phone. Installation of a custom ROM is easy if you learn the basic of it. If you don't know how to install custom ROMs using TWRP recovery, see our tutorial on How to Install Custom ROM using TWRP!