HowTo
From Richard's Wiki
To extend the ring time simply enter * * 61 * 1 0 1 * * (number of seconds: 15, 20, 25 or 30) # and press SEND or CALL. For example, to set the ring time to 25 seconds before it diverts to MessageBank, you would enter the following key sequence into your phone: * * 6 1 * 1 0 1 * * 2 5 # and press SEND or CALL. If you set the ring time to 30 seconds and find this is too long, you simply repeat the process with the desired length. If you are wanting to re-set the ring time to default (15 seconds) enter # # 0 0 4 #