See the AT&T article for instructions on how to forward your phone to another phone number. Note, this does use your minutes and if you forward to an international number you pay the fees.
How to Work Around the Bug in AT&T iPhones
There is a bug in one of the steps in that the icon will never appear. If you press the "<forwarding" link in the upper left, the phone number will disappear and it will not set. Use the work-around below to forward the phone if you encounter this issue.
- From the Home screen, tap Settings > Phone > Call Forwarding
- Tap the Call Forwarding toggle to set to On and a new box will appear for the phone number
- Enter the desired forwarding number
- Press the hardware home button on the bottom of the phone
- Again go to the Home screen, tap Settings > Phone > Call Forwarding again and you will likely find the phone number was saved and the call forwarding feature is turned off
- Press the toggle button to turn on the call forwarding
The instructions to disable the call forwarding are correct. The instructions to change a number are likely also not going to work and you should, if needed, follow the work-around above.