Straight talk on AT&T iPhone 3GS?

I currently am using an iPhone 3GS with an AT&T sim, and a GoPhone plan. Is it possible to switch to a StraightTalk plan and continue to use my iPhone? Do I just replace my AT&T sim with a StraightTalk sim? Would I need to buy the "Straight Talk AT&T Compatible Sim Card" I saw on Walmart.com?

Added (1). I'm not sure if this phone is "unlocked" or what exactly that even means. It was purchased from EBay as an "AT&T iPhone" which I put my very old AT&T GoPhone sim in. The phone has never been tied to a contract.

Added (2). I also do not use data. I talk, text, and use Wi-Fi only.

Yea, but it sucks because you will have to youtube a video to find out how to get the data to start working on it, and when you do get it working, you only get 2gb a month

I'm currently using an iPhone 3gs on straight talk that had been used on AT&T prior. It is updated to iOS 6 currently and isn't unlocked. I did order a sim card from straight talk.com use the code simsale or simsave to get either 5 or 10 off not sure which is which and it expires soon, free shipping as well.

As far as data working I just went here using the Wi-Fi on my phone http://www.unlockit.co.nz and followed the directions and it changed the apn settings for me, but you may need to turn the phone on and off and wait a bit before everything works, but it all will work eventually it did for me. Another thing people have done is to use a simple mobile or t-mobile sim to swap and trick the phone into opening the apn settings for you to change.

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