The foolproof way by the1law:
First off, remove the the unlockit.co.nz APN from the iphone. If you didn't download an APN then skip this step. If you don't remove it will screw it all up every time you try to change the APN settings. I forgot where it is but it's somewhere in settings, general... maybe under Cellular. It will say delete APN in red. Or you can reset the network settings to factory. Settings, general, reset, reset network settings. I would advise to do both.
1. Double click on the home button and close every single app.
2. Open the Notes app
3. Go to Settings > General
4. Insert the T-Mobile SIM (can be brand new/unactivated OR an old one that has had service) will bring up the Cellular option
5. Go into Settings > General > Reset > Reset network settings. Reset the network.
6. Go into Settings > General > Cellular > Cellular Data Network when it shows up
7. Immediately take out the T-Mobile SIM and put the Straight Talk SIM back in while in the Cellular Data Network tab
8. Multitask between Cellular Data Network and the Notes app back and forth until the default non-working data & MMS APN are automatically entered. It is important to wait until the default APN settings show up
For those of you who don't know, click the home button twice to multitask between the two.
9. Once the APNs do show up, edit them manually
Cellular Data
APN: att.mvno
Username: *blank*
Password: *blank*
MMS
APN: att.mvno
Username: *blank*
Password: *blank*
MMSC:
http://mmsc.cingular.com
MMS Proxy: 66.209.11.33:80
MMS Max Message Size: 1048576
MMS UA Prof URL:
http://www.apple.com/mms/uaprof.rdf
10. Go toggle Airplane Mode on and off, and then turn your iphone off and then back on
After Step 10, check that your data & mms ARE working. Then turn off your phone one more time and then back on again. Check data & mms again. Data should work but mms might not. If it doesn't, try this; go into Settings > Cellular > then toggle Cellular Data off and back on and everything should be working.
If it's not working for you, reset the network settings and try again.