By default, the iPhone rings 15 seconds before going to voicemail. I wanted to make this a little longer so that I could use the Vonage SimulRing feature without my cell phone’s voicemail answering before the Vonage voicemail would answer. Unfortunately there is no setting I could find in the iPhone interface to change this, but luckily I found a forum post explaining this.
1. Figure out your voicemail access number. Dial *#61# and then press Call on your iPhone’s keypad. The first block of text you’ll see on the screen that comes up will be:
Setting Interrogation Succeeded
Voice Call Forwarding
Forwards to +1XXXXXXXXXX Enabled
Where +1XXXXXXXXXX is shown as an actual number – write this number down because you’ll need it in the next steps.
2. Dial *61*+1XXXXXXXXXX*11*tt# where +1XXXXXXXXXX is the number you wrote down from the first step above and tt is the length of time in seconds for which you want your iPhone to ring. To get the + sign to show up, hold down the 0 key for a couple of seconds and release it when it turns into the + sign in the readout above. Then press Call.
This has been verified to work up to 30 seconds. I set mine to 20 seconds and my Vonage SimulRing to 15 seconds – that seemed about right.
Thanks to macgirl who originally posted this at http://www.everythingicafe.com/forum/iphone/how-to-change-how-long-your-iphone-rings-before-calls-go-to-voicemail-19389.html