Welcome to the community!
You can try this method it usually works.
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|
If this method does not work for you then you will need to contact customer service and have them do it on their end. You can reach out to them through any of the available methods listed here.