For 8X8 you need to log into your account manager and go to the phone system. Once in the phone system, you need to click view all extensions. Find the extension whose forwarding rules you wish to set, then click Edit to the left. Scroll down to the Call Forwarding section and click the + sign to expand. Review the Forward Type (conditions in which a call is forwarded) and forwarding destination columns. You can decide to always forward under any circumstance, or you can configure rules for Busy, No Answer, and Internet Outage individually. When your rules are set, click Save Changes at the bottom left of the page.