Can you bring your own iphone to AT&T? (Please LOOK AT THE LINK AND TRY)?

I bought an iphone 4s on craglist and it was on AT&T and I unlocked it then got a sim for T-mobile. Tmobile really sucks with their internet and data. I want to transfer to AT&T and I read online that you can transfer to a month to month plan buts its not the go phone plan. If you need a login mine username is pooo and my password is pooooo. Thanks for the trouble.http://forums.att.com/t5/Apple-Community-Discussion/No-Contract-AT-amp-T-Voice-Data-Plan-for-iPhone-purchased-used/td-p/1895360

You can, I bought an iPhone and I was able to get a sim card for free with AT&T. Just tell them you want to transfer and need a sim card and they'll probably only charge you to start a data plan.