Since I'm picky when it comes to my phone, not having full MMS functionality did not sit well with me. The solution I came across was to use a t-mobile sim to activate the cellular data settings. The process I used is detailed below.
First some preliminaries:
- Get your iPhone factory unlocked. Either through AT&T here or by purchasing a factory unlock service on eBay. I had to do the eBay unlock service because the AT&T unlock didn't work for me. Either way, once one of those unlock methods work, you'll need to restore you phone in iTunes for the unlock to be effective. It will tell you that your phone is unlocked when the restore completes. I believe it is required in order for the Cellular Data Settings to come up when you use the t-mobile SIM later.
- Get an inactive T-Mobile micro SIM. eBay is great way to get one, just make sure it is a micro SIM, not a standard size SIM. Otherwise you will have to cut down the standard size sim to micro sim size, so save yourself the trouble and purchase a MICRO SIM.
Now for the actual process:
- Turn off wifi. Keep your existing APN configuration profile in place. It's good to have it there just in case the APN settings reset randomly. This has happened to me a few times.
- Open the Notes app so that it is in the background for later.
- Insert T-Mobile SIM into iPhone. Wait a few seconds and go to Settings > General > Cellular > Cellular Data Network. If you don't see the Cellular Data Network menu item, it might be because your phone is not unlocked.
- Switch to Notes app. Do this by double tapping home button and select the Notes app. Stay in the Notes app.
- Insert Straight Talk SIM. It's important that you stay in the Notes app for at least a minute as different things flash up in the upper left corner of the screen. Wait till it says HOME and stays on HOME.
- Switch back to Settings. Do this by double tapping home button and selecting Settings app. Settings should still be on the Cellular Data Network screen.
- Enter Straight Talk settings. The settings here should do the trick.
- Go back to General. Hit the back button in the upper left corner to get back to the General screen. Your settings should now be saved.
- At this point your phone should be good to go but you may need to reboot the phone for everything to take effect.
Some other things to think about:
You may find that updating to a new version of iOS, even if it is only a minor version upgrade will reset the settings. If that happens, just follow the process above again. Also, I would be wary about upgrading to the next major version such as iOS 7. There's no guarantee this workaround will still work.
Hopefully this process works for you, it did for me. However, if it didn't, you can always revert back to installing the Straight Talk profile from http://www.unlockit.co.nz/unlockit/ and still have data & MMS iPhone to iPhone.
As of the iOS 7.0.3 update my Straight Talk SIM just works, no APN settings or config profiles needed. I still keep the APN config profile installed just in case. This is for the newer LTE SIMs, I don't know if this works for the older ones.
Straight Talk has AT&T compatible SIMs for sell now.