You may simply need to adjust the number of rings (or beeps) as per this instruction: How do I edit my notification tones (profiles)?
If however, you do need additional ring time before voicemail:
A) The simplest method is to call your carrier and request a longer ring time, up to 30 seconds.
B) If you have a GSM based phone, some have had success by dialing this number from your phone: **61*XXXXXXXXXXX**30#
- Replace XXXXXXXXXXX with the number of your carrier voice mail center (find it by looking at your Phone Dialer > Options > VoiceMail > Access Number. Put your access number in place of the Xs above.
For example, the T-Mobile voice mail access number is 18056377243, and thus the string to dial would be **61*18056377243**30#
- Replace the 30 at the end of the string with your desired seconds. For most carriers, 30 seconds is the longest allowed.
- NOTE: Has been reported this doesn’t work for Rogers (8700r) . 6/17/2005. Morser