Unlock iPhone 4s to T-Mobile?

I'm thinking about buying an iPhone 4s locked to AT&T. I'd like to get it unlocked so I could use it on T-Mobile's network, but I'm not sure how the process works. I know how to Jailbreak, but will that unlock it? Or can I call AT&T and have them factory unlock it for me? Or is there a reliable company that does unlocks?

When we wanted to use an at&t iphone 4 on t mobile, we had to have the original iphone owner call at&t to request to have it unlocked. They gave her instructions for the process which takes a few days as they have to be sure the phone's been paid off by fulfilling a 2year.contract. After they figure it out, they email the owner who follows their instructions to unlock and reset it to factory specs.

Then you can take it to another provider, like tmobile, and they treat it like a new phone.