i think that it just defaults to play a "sound" or ringtone for at least 30 seconds. I had a ringtone that on iTunes was only 15 seconds long that went "ringring ringring", but when i get a call it is "ringring ringring ringring ringring".
I believe that's done through your voicemail settings. I'm a little foggy as to how it works properly these days, but I used to get there by calling my own number then dialing * and my voicemail password as soon as the voicemail picked up. I just tried that now and it's a little flaky, but it got me to the main menu. I'm not going through all the options right now, but I'm pretty sure what you want is in there.