Have you read the users guide? Did you do a SEARCH?
The simplest method is to call your carrier and request a longer ring time, up to 30 seconds.
Not sure if this works on the 82xx series as I have not tried it. Works on other BBs however.
Dial *94 followed by the number rings you want before voice mail initiates (it has to be between 2 and 9), then SEND/TALK