The easiest way would be to call your provider. They can easily fix your issue. If the CSR says they cant ask for a more capable CSR or tier 2 support. If you can, try a search on voice mail. There was a post a while back that also gave a code to enter to extend the number of rings. But IMO the easiest way is to just call your provider.
Cingular recommends not extending the ring time beyond 25 seconds to avoid being charged for unanswered calls. I have an 8700 and the default time was 20 seconds. I contacted Cingular and they increased the time to 25 seconds with no problem.