How to unlock an iPhone 4 from it's carrier?

I have an iPhone 4 that's on AT&T and I want to unlock it to T-Mobile. It's non jailbroken though.

I heard that you could unlock it someway by installing a source from Cydia. I have jailbroken my other iDevices except for my iPhone, so I'm familiar with Cydia and how to use it. Please give me step by step instructions on how I can unlock my iPhone from it's carrier

The carrier unlock only works on jailbroken phones. The terms "jailbreak" and "unlock" are often confused, but mean very different things. A jailbroken phone allows the user to run third-party apps that are otherwise not approved by Apple. An unlocked phone allows the phone's owner to choose a different carrier. Apple has an exclusive contract with AT&T to offer the iPhone in the US, meaning iPhone owners can't chose their own carrier. This "carrier unlock" just released by the "iPhone Dev-Team", allows users to switch to the carrier of their choice.
To unlock your iPhone, it must first be jailbroken, which just became drastically easier with this week's release of a browser-based jailbreak that literally takes seconds to complete. Once the phone is jailbroken, the carrier unlock is available in the Cydia app directory.
check the safe and easy unlock guide for iphone 4:
http://unlockanyattiphone.com/

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