You'll want to log into your member tools customers portal here:https://members.sonic.net/
On the left you'll see a menu. You'll want to click on "Voice" and then click on your number.
Once you do this you'll see the commportal phone options. Click on "Open Call Manager". Under this setting you'll want to change "Send to Voicemail After" from 15 seconds to at least 36 seconds. This is typically around 4-5 rings then click "Apply" in the upper right corner when you finished. I hope this helps. Let us know if you have any other questions. Have a great day!