Each call forwarding number can have a custom number of rings before your voicemail picks up. To change this, click Extensions, then choose the Extension you want to change. Next, click Edit:
You’ll see the “Ring For...” setting that you can change to as low as 20 or as much as 120 seconds:
By default, this "ring for" time is set to 40 seconds, since that seems to be the average amount of time it takes for people to pick up.
If you signed up before June 2011, follow the instructions below.
Go to: Settings > Extensions > Select the extension you want to edit the ring time for > Select Call Forwarding (right hand menu) > Under Details click the down arrow > Enter the time you want us to try your number for under Seconds before going to next number