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It'll work without unlock as long as it's an AT&T
iPhone with an AT&T Straight Talk SIM. Also, same applies to T-Mobile. T-Mobile iPhone works without unlocking on T-Mobile Straight Talk SIM.
Correct.


Did you go to unlockit.co.nz to download a profile for Straight Talk? This will allow data to work but not MMS. Or the SIM swap trick would work in order to change the APN settings manually.
With the iPhone 4 running iOS5, it was possible to get MMS working without jailbreaking or SIM swap.

I don't know if this info is still valid for newer iPhone/iOS combinations but this is the process that I used on my iPhone 4 running iOS 5.x:

http://www.howardforums.com/showthread.php/1760217-How-To-AT-amp-T-locked-iPhone-4-MMS-working-on-Straight-Talk-no-jailbreak
 
You missed the $2 sim sale from T-Mobiles website, but you can pick a sim up on eBay or Amazon for under $5. Just make sure it's cut to Nano Size (or an actual Nano Sim), if you don't have a sim cutter.

I just ordered a T-Mobile Sim on Amazon for $3 and a Straight talk sim for $7 (on sale) so it was just about $10 :)
 
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