With many carriers it is easy to change. With AT&T, you dial:
*61*XXXXXXXXXX**N#, where XX's are your phone number, and N is the number of seconds (from 5 to 30) you want it to wring before going to voicemail.
I have a contact setup called answerlength. I have two phone numbers in it, one with the above setup for 10 seconds, and the other with 30 seconds. When I am in a meeting or movie, and I have the phone set to silent or vibrate, I use the 10 second ring to voice mail. I have these numbers assigned to keys L (for long), and S (for short). Then I just hold the key down for a few seconds and it changes my ring to voice mail length. Other codes:
GSM NETWORK CODES