By default, the iPhone will forward to voice mail after 20 seconds if you don't answer your phone. If you like, you can change this value in one of two ways. The first way is to call AT&T directly and sit on hold for a long time. Once you get a customer support rep, they can change this value for you. If you are feeling more adventurous, you can also change this value yourself through the following procedure:

1) From the iPhone keypad, make a call to this number: *#61#

2) This will forward to your voice mail. Write down the number that this forward to... it's shown as the number after "Forwards to".

3) From the iPhone keypad, dial the following number: *61*1<voice mail number>*11*<voice mail delay in seconds>#

In the above number, substitute your 10-digit voice mail number for the <voice mail number> text and substitute your desired delay for <voice mail delay in seconds>. In both cases, make sure you also take out the < and > symbols. Here's what an actual number might look like if you were calling from a movie: *61*18005551212*11*30#

In this case, the delay would be increased from 20 seconds to 30 seconds. 30 seconds is the maximum delay allowed, so don't bother entering a larger value.

Once you call this number, you should see something along the lines of the following on your iPhone:

Setting Activation Succeeded
Voice Call Forwarding
When Unanswered

Congratulations, you have changed the amount of time your iPhone waits before forwarding callers to voice mail.