You will need a T-Mobile sim. It does not need to be activated. However, phone may need to be unlocked. Try this after obtaining a T-Mobile sim.
I have done this and can verify it works.
I also have the same problem but I have a 3g and a 3gs. 3gs 6.0 3g 4.0 ?. I have a att pre pay card 10 cents a minut plan and when I put the sim in thr 3g mms works fine. If I take the same sim and put it in the 3gs mms does mot send of recieve. My 3gs worked fint till I updated it from 5.1.1 to 6.0. go figure.
Heres a simple guide on how to fix your MMS picture messaging on you iDevice WITHOUT THE NEED OF A JAILBREAK!!!
Go into SETTINGS/GENERAL/CELLULAR DATA NETWORK
MMS PROXY: 216.155.165.050
MMS MAX MESSAGE SIZE: 614400
MMS UA PROF URL: http://wap.sonyericsson.com/UAprof/W800iR101.xml
Internet apn: epc.tmobile.com
After searching the internet for quite a while, I finally found a fix that requires not much effort, no jailbreak, and no cost.
I have an iPhone 4 that I just converted from AT+T to Straighttalk. My number transferred easily enough in a few hours, and my cell phone made calls.
Next, to get my data operational I connected via Wifi on my iPhone to the site http://www.unlockit.co.nz
This site very easily and quickly set my data APN correctly by choosing Straighttalk from the list.
(By the way, it also downloaded a small Unlockit web app to my phone, which I later deleted.)
Finally, to get MMS working, I found this useful video: http://www.youtube.com/watch?v=Te5R9HnB8kY&hd=1
You download the program "ibackupbot" from this website: http://www.icopybot.com/download.htm
(There are both Mac and Windows versions)
It also is helpful to have the full text instructions: https://docs.google.com/file/d/0B8jUv4GOCA6DOWRsZEwtQkFFMjA/edit
In a nutshell, what this workaround does is:
1) You make a computer backup of your phone in iTunes
2) The program ibackupbot allows you to edit individual .plist files in the backup
3) Then you restore your iPhone from ibackupbot directly.
I just completed the process and successfully sent a picture MMS!
There was a moment of fright after restoring the phone. It rebooted (as it is supposed to) but failed to reboot twice! Just got the white Apple, then it shut down after a few seconds. The third time, however, I got a progress bar and it rebooted correctly.
There are some comments indicating that these changes don't "stick" for long. I'll update if I find this to be the case. Fingers crossed.
FYI For those of you who don't have an unlocked phone, I was finally able to get my LOCKED iPhone 4S AT&T gophone back in business using the T-mobile SIM and gamerdude's link. It took a few tries and I had to be pretty fast with the swap, but it definitely works.
Just be sure (as many people have mentioned) to only use the T-Mobile SIM to get to the Cellular Data Network screen - then immediately pop it out and put your At&T SIM back in. It shouldn't boot you out of that screen, giving you a chance to enter the settings. But make sure you enter everything exactly as the unlockit.co link specifies - once you exit out of that screen, it's gone forever... Thanks a lot, AT&T.
My wife upgraded her 3GS to from 6.0 to 6.01 so I had to do this again.
I decided to try a different method and it worked fine.
- booted with the ATT SIM
- swapped to the T-mobile SIM without rebooting (this was an activated and working SIM)
- made the required changes per the previous posts
- swapped back to the ATT SIM
All done without any reboots and it worked great.
Thank you so much for your information and excellent instructions! I just completed this on my Iphone 4 on iOS 6.1 (upgraded yesterday) with complete success. With just a little effort and close following of the instructions to a "T", and I'm up and running 100%, talk, text, and pic text on Straight Talk with my previous AT&T phone in a short amount of time. Awesome!