Shotening Rings When iPhone is On

Try this... at your own risk though. Found it via Google...

1. enter #6111# and press send/talk on your phone.
2. write down the number your calls are currently forwarded to (usually +1401#######, but not always). this is your voicemail deposit number.
3. decide how long you want the phone to ring in 5 second intervals. the maximum time is 30 seconds. i'd recommend 25 seconds or less, as using 30 may cause you to be billed for a minute of airtime each time someone is forwarded to voicemail.
4. now enter *61[number from step 2] 11[number from step 3, use 2 digits]# and press send/talk.

example
voicemail deposit number is +14015551212
you want it to ring for 20 seconds
you would enter *61*+14015551212*1120# and press send/talk.