On your phone's keypad
First dial *#61# this will show you your voicemail forwarding number
Then dial *61*voicemail forwarding number**number of seconds#
The number of seconds must one of these numbers: 5, 10, 15, 20, 25, or 30
|Ring time:||Number of rings:|
|10 seconds||3 or 4|
|20 seconds||4 or 5|
|30 seconds||5 or 6|
Welcome to the community!
I believe changing the number of rings can only be done by customer service (see here). Another community member posted a ring code you might try, however, that method doesn't appear to work for everyone. If that method doesn't work, the change would have to be done by customer service. In addition, they can also be contacted via Live Chat, Facebook, or Twitter. Those methods can be accessed via the contact page posted above.
Hope this helps 😀