I found this very direct solution on the AT&T Wireless discussion boards. I just did it and it worked on my iPhone 3G:
1. On your phone, dial *#61# and click Send.
2. Some information should be displayed: number that the calls are being forwarded to and the delay before the forwarding engages.
3. Write down the number (including 1)
4. Dial
*61*1xxxyyyzzzz*1130# and hit Send. 1xxxyyyzzzz is the number you wrote down previously, 30 is the delay in seconds. The delay can be set in 5 second increments, 30 is maximum.
5. Dial *#61# to verify that the new settings are active.
The original post was here:
http://forums.wireless.att.com/cng/board/message?board.id=voicemail&thread.id=83 66