How to unlock iPhone 4 baseband 04.12.05 ios 6.1.3?

I bought this iPhone from a friend right before he moved and he doesn't have a receipt and and it's lock to AT&T. But for them to unlock you need a receipt and I called my friend and he dosent have it. Please is there any way I can unlock it I want to use tmobile.

Yep! Just go to http://phonerebel.com/unlock-any-iphone/# its only about 5$ and i've done about 3 phones that are locked to att.