If you want MORE time until the voice mail picks up... Increase amount of rings before voicemail - BlackBerryFAQ
Which will give you more time... time = rings.
If you set a 5 second ring... you are getting about three rings now 5*3+15 seconds normal ringtime before voice mail. You can extend that time to 30.
If you set a 12 sec ringtone, you will get 2 full rings... so the number of rings is determined by your ringtone length.