How to use a AT&T iPhone 4 on T-Mobile?

I have a tmobile account and am looking to buy an at&t iphone 4. How simple is it to use the phone on tmobile? I've done my research and it looks like at&t can unlock your iphone after you are done with a contract. But I don't have that. I don't want to jailbreak the phone, but is that my only option at this point?

I thought you can bring an iPhone to T-Mobile,
and they will unlock it for you, for their network

Luckily it is not the only option for you. You can unlock unlock your AT&T network locked iPhone 4 safer and easier through remote unlocking method. You can get the remote unlocking service for your iPhone 4 from http://www.unlock-zone.com/apple-iphone-4/rs5wp9/ at low cost. This method of unlocking doesn't require a Jailbreak too as you expected. You can also visit http://www.mobileunlockguide.com/apple-iphone-4 this site provides free unlocking instructions. This could help you understand remote unlocking service better. All the best mate.

You can get factory unlocked iphone, then you do not need jailbreak it.

With the launching of iphone 5 you can now unlock all iphone 4 & 4s from att here
http://www.vunlock.com/
they are perfect for this

Maybe you need to use unlock method that my friend used to unlock his iPhone. My friend unlocked his iPhone through this website:
http://officialappleunlocker.com

their price was about $25, so cheap in comparison to some others there and they unlocked my friend's iPhone in 12 hours, also they just got imei # and unlocked it.take a look at their website.

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