For all of you that want a real answer here ya go. This isn't something you can change on your phone this has to be changed with your service provider. If you are with AT&T, like I am, then this is the solution for you (link of where I found it below as well).
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*11*20#
and press send/talk.
if you have trouble with the above procedure, you can always call customer care at 800-331-0500 to change it.
QUESTION: How can I adjust the number of times my wireless phone will ring before the caller goes to voicemail?
ANSWER: Contact Customer Care at 1-800-331-0500 or dial 611 from your wireless phone to change the amount of time your phone will ring before going to voice mail. The adjustments made by Customer Care are based on the traditional ring heard by the caller, not the unique Ringtone you hear from your phone.
I just wanted to add that increasing the ring length to 30 seconds will not cause you to be charged a minute should the caller go to voicemail. This was confirmed by an AT&T technician.http://forums.att.com/t5/Voice-Mail/Increase-the-number-of-rings-before-a-call-goes-to-the-voicemail/td-p/2576651