Which software/website should I use to factory unlock my AT&T iPhone 4S? - 2

I need to unlock it to be used on T-Mobile. I want a permanent unlock and I was wondering if any of you have done it and what your suggestions are? Thanks (:

Added (1). I'm buying it from a family member. I have never had AT&T, always T-Mobile.So, AT&T won't unlock it for me. I wanna know a good website that does it.

The original carrier can unlock for free if your off a 2 year contract, or you will need to pay $20 to have it unlocked.
i can unlock it

If you've had the phone for 2yrs. Then you've paid for it and at&t will unlock it for you. You have to call them and they give you instructions on what you have to do. Follow what they tell you and if it's paid off, they will unlock it.

It takes them a few days to check the phone out and send you an email with instructions to do the unlock. When you are done, it's ready for a new carrier.