AT&T will change it for you. There is also a GSM code that you should be able to enter from your phone to change it yourself.
From the AT&T Forum (link
The max time is 30 seconds. You can either call customer service and have them set the number of seconds or you can use these instructions: |
You can use the following codes to control the delay from 5 to 30 seconds on any GSM phone:
- To get the current call forwarding status, enter *#61#[SEND] into your phone.
- Write down the 10-digit number(ignore the +1), this is the phone number that your calls are currently being diverted to.
- Enter **61*[10 digit number]*11*[delay]#[SEND] into your phone using the phone number (numbers only) as reported by the status & the delay can be set to (5, 10, 15, 20, 25, or 30) seconds.
- To verify that the settings are correct, enter *#61#[SEND] into your phone.