The amount of time before a call goes to voice-mail is determined by the carrier, not the length of your ringtone. If you want your phone to ring for 30 sec before going to voice-mail and your in the US, call AT&T and have them set the ring time to 30sec. To make an existing ringtone longer, would require editing in a program like garage band, if your on a mac. On windows, you need an editor like Audacity.
http://audacity.sourceforge.net/
When I make ringtones, I set the length to 29 sec., just to be on the safe side.
