Yes, You need to unlock your AT&T iPhone 3GS to use with T-Mobile. You can unlock your iPhone 3GS safer and easier through remote unlocking method. You can get the remote unlocking for an iPhone 3GS from www.unlocking4u.com/apple-iphone-3gs/rs6wp9/
at affordable cost. This site is a legit source to rely on accurate unlocking solutions.
Here are the steps for unlocking an iPhone 3GS through remote unlocking method,
1. Wait for us to send you an email saying your phone has been activated or remotely unlocked.
2. Install the latest version of iTunes.
3. Make sure your Apple iPhone 3GS phone software is updated to the latest version using the iTunes software.
4. Connect your Apple iPhone 3GS to iTunes with a NOT ACCEPTED
5. Wait until iTunes detects your Apple iPhone 3GS.
6. Disconnect your Apple iPhone 3GS and reconnect it after 10 seconds.
7. Your Apple iPhone 3GS is now unlocked.