Call forwarding via a specific number is the way we previously had users setup and run call forwarding. However Ring Groups have become the primary way users should setup call forwarding in the future.
To setup a ring group you'll go to SETTINGS > PHONE DIALER > RING GROUPS. Enter a name for the ring group on the right > Add Ring Group button.
Once created you can edit the settings for the ring group. You can set another group to be who get's rung should the newly created ring group not answer, as well as changing the maximum phones that can be rung in a ring group at once. The max is 10 at once. You don't want to set this number too low or incoming calls me be left ringing for extended periods of time.
At the bottom you'll add in the phone numbers/users you wish to add to the ring group, you can add users in Radius who are not yet part of the ring group or type in an external number outside of Radius to be added to the ring group. Once selected in the dropdown use the add phone button to add the user or phone number to the ring group > save.
Once the ring group is created and everyone is assigned as you wish you'll go over to the phone numbers tab and add the ring group by using the blue button next to the number you want to assign the group to > edit > select the ring group from the "Direct Calls to User or Group" dropdown > save information.