Click the 9 square button at the top right of the screen.
Click on Callflows, then click on Callflows once more.
Click Add from the top left.
Give the directory a name.
From the Add User section on the right, set the User AND callflow via the drop-down menus, and then click the Add this user to the directory link.
Note: A callflow MUST be set, but it does not have to be the default callflow for that user. (Example: A caller wants to talk to the CEO, you can set the callflow to actually ring their assistant by selecting that users SPBX callflow from the menu)
Once all users have been added to the directory, click Save.