So, here’s an updated method on how to root your T-Mobile Galaxy S2 SGH-T989, this root method will work on both Gingerbread, ICS, and Jelly Bean. (Works on all Android 2.3.4,2.3.5,4.0.3,4.0.4,4.1.1,4.1.2)
*Note – This method is for Windows only, Heimdall does not support T-Mobile Galaxy S2.
*Note2 – This method also works for SGH-T989D
UPDATE: You can also use this method to root AT&T Galaxy S2 Skyrocket SGH-i727 by using a different tar file for Step 6! (See Step 6!!!)
UPDATE: This root method works regardless of Android OS version, works on Jelly Bean.
Step 1. First, plug in a micro-USB cable from your T-Mobile Galaxy S2 to your computer then hold down Volume Up, Volume Down, and the Power button together for about 10 seconds.
Step 2. When your phone resets (you will feel a little vibration), let go of the Power button but keep holding down the Volume Up and Volume Down buttons.