Can you unlock and jailbreak a used iPhone that was on AT&T?

[iGeeksElectronics]

Hello all, thank you for viewing my question. I was wondering, I am curious buying an iPhone for Christmas.. But, I have T-Mobile and have heard you cannot UNLOCK and Jailbreak a used iPhone that's already been used and activated for AT&T service, now is this true? Thank you guys and please I do not want any immature kids jumping in saying 'omg that is bad', if you have NOTHING TO DO with this QUESTION, please leave.

[HoofArted]

Go here to download: blackweather

Go here for easy instructions and info: On the iPhone

[geeth]

Hi no problem you can easily unlock iphone from AT&T network at Onlinegsmunlock.com here they help you unlock your iphone using remote unlocking method with out jail breaking it .I hope it will be helpful to you .