Here are the APN settings you should use for Straight Talk. You will need to change these to get data and MMS on your iPhone:

NAME: straight talk
APN: att.mvno
PORT: 80
MMSC: http://mmsc.cingular.com
MMS PROXY: 66.209.11.33
MMS PORT: 80

Change the APN settings on your iPhone:
Settings > General > Network > Cellular Data Network

If you cannot change these settings then you need to unlock your iPhone.


See More: Change the APN settings on your iPhone for Straight Talk