I bought a used att iphone but want it unlocked?

I bought a used att iphone from a friend but i want to unlock it so i can use it for tmobile, the 3rd party unlocking service went down though so i'm not sure how i can get it unlocked. I heard to file a device unlock from att but how do i do that? Can i ask a friend?

I unlocked my iPhone 4S on this site: unlockiphoneforfree.tk Everything is smooth and bug free.
Goodluck!