With your iPhone you can get free text messages regarding various aspects of your service. You can check your balance, the amount of minutes used, and the amount of data used. Don’t forget that you also can email to SMS for free with the iPhone.
Calling these numbers on your iPhone will get free text messages sent to you with a lot of helpful information.
To check minutes dial: *MIN# or *646#
To check your balance dial: *BAL# or *225#
To check balance of a prepaid phone: *777#
To check data use and number of text messages sent dial: *DATA# or *3282#
Some of these can also be found in the following area of your iPhone:
Main Menu -> Settings -> Phone -> AT&T Services
If you still need to SMS and you are running low on messages, remember that you still can send free outgoing emails. By using common email to sms codes, you can conserve some of your text messages as well.