The following is what you will need to do this:
T-mobile Sim Card Noosy Sim Cutter iPhone 4S Pin
Here is how you do it:
These instructions are written using iOS6. You can only use the following methods if your iPhone supports multi tasking.
You need to be able to access the Cellular Data Network tab in Settings>General>Cellular. When you have the Straight Talk sim in the phone it will not appear. This stops you from being able to set the correct MMS APN settings on your phone. How to get around this is by using a un-activated T-mobile sim card.
I got my T-mobile sim card from a prepaid phone that I bought on sale at Walgreens for $14.99. I then took the sim card out and used my Noosy sim card cutter to get the card to micro size. You could use scissors to do this but it is much easier to use the sim card cutter.
Take your ST micro sim card out of your iPhone 4S. Then place your newly cut micro sim card from T-mobile inside your iPhone. Go to Settings>General>Cellular (sometimes you have to wait till' Cellular activates) and then Cellular Data Network will appear and tap into it. Congratulations! You have successfully made it to the APN settings area. First though, double click your home button and bring up the multi-tasking submenu on your phone and go to another app. If you don't do this quick your iPhone will realize that your not using a activated sim and go back to the activation screen.
After you've successfully multi tasked over to another app, (even if the phone goes back to the activation screen) take the T-mobile sim card out and put the ST one back in. Once your phone has registered that your back on Straight Talk you need to hit the home button twice and switch back to settings. It has most likely placed random settings in the APN under Cellular Data Network. You will want to put the following settings in for the APN.
Data APN att.mvno MMS APN att.mvno MMS MMSC http://mmsc.cingular.com MMS Proxy 184.108.40.206:80
Once you have entered these settings double click the home button to open the multitasking menu. Switch over to another application and then switch back to the settings app. You must do this using the multi tasking menu by double clicking your home button. Once you switch back to the settings app and the APN is the same then you know it worked. Go back to the home screen and test your data and MMS. It should work!
Alternative: When switching out of your settings app you could go into your Messaging app and test a MMS. If it fails you could always switch back to your settings and check your APN settings.
If you need any help with this contact: email@example.com or leave a comment below.