I believe that option was taken away when they updated the voicemail platform. You can contact telus below and double check with them. Live chat is your best option for a fast response (bottom right corner). Unless @Mobility_Princess knows off hand.
Their are a number of previous responses found by typing "Adjust voicemail rings" in the Search Telus.com box above on the right. The answer seems phone dependent.
It's very dependent on a lot of factors, type of voicemail in your plan, type of phone you have. It can easily be changed by calling/chatting Telus. You will want to call from a different device as you will have to do some things on the phone itself.