IPhone 3GS on t-mobile service?

Hi! So I have a iPhone 3GS that used to have AT&T service, is it possible to have tmobile service on it? If I need to "unlock" the phone, what do I have to do to unlock it? Thanks!

Yes, absolutely! Take your phone to T-Mobile and ask them to flash it over to T-Mobile from AT&T. Both carriers use GSM. If you just want a fully unlocked phone- for say international travel- your best bet is to look on Craigslist and get someone nearby working out of their home to do it for really cheap.

You will need to unlock it. You can unlock it using iphonunlocker.com is the best software on the market.

Yes you can unlock it. You need to get an unlock service to have it unlock. Once your phone has been unlock, you will be able to use it to different carriers.