Cell Minute Tracker  ($0.99) Helps you keep tabs on your cell phone usage, data plan, rollover minutes, SMS costs & payments in a native iPhone Environment. The information is shown on one compact page.

You’ll need to register for a MyAccount login from AT&T.com for Cell Minute Tracker to work. The app talks directly to AT&T’s online minutes-tracking service & displays the information on the compact page hence bringing a native experience to your device. 

Note: You can check you minutes by going into settings>>Phone>>AT&T Service>>View my Minutes.  You will get a free sms showing you the usage.