Call availability matrix enables you to define the availabilities on users on days of the week along with the fallback users if they are unavailable. 

This feature is super easy to use. As shown in below screenshot, you can view the availability of users day wise. 

With the easy display of call availability in matrix form, one is able to mark a user unavailable and can easily decide which fallback user can work instead of unavailable user.  You only need to check which users are available on the day and make one/more from those as fallback users.

In order to check details of who are the fallback users for certain unavailability, you can click on information icon.