I found this recently on an AT&T forum post on how to change the amount of time your phone waits before rolling over to voice mail:
- On your phone, dial *#61# and click Send.
- Some information should be displayed: number that the calls are being forwarded to and the delay before the forwarding engages.
- Write down the number (including +1)
- Dial **61*+1xxxyyyzzzz*11*30# 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
- Dial *#61# to verify that the new settings are active.
The crucial detail is the “+” - you have to figure out how to include the plus in the dialed number. On my (8525) phone, I hold down “0″ for a while and it turns into “+”. I hope this helps someone else.