The following is a step by step guide to unlock AT&T Samsung Galaxy S (Captivate) to use on All GSM carriers including T-Mobile. It is fairly long process but I have verified every step on both Android 2.2 and 2.1 and it had worked for me. It should work for you too. This is the easiest method i've found and it does not require you type or modify files manually. So it's safest and you will be unlikely to brick the phone.
- Check whether your phone is already unlocked or not. Follow the steps here.
- If it is not unlocked. Download and install superoneclick to root the phone. A good instruction video below. You can download the tool from here.
- Now go to Marketplace download and install the busybox Installer by Stericson. Run it to install busybox.
- Now go to Marketplace download and install the app Galaxy_S Unlock by Helroz. Follow the steps in the app to unlock. A good instruction video below:
Note: If the anything goes wrong. You may hard reset the AT&T Samsung Galaxy S and start over.
Leave a comment if it works for you!