Check that the function switch you want is activated. This is shown by the switch turning blue.
To register an internal "telephone network", you must provide a name and a telephone number (Or AppleID).
Design of individual activities
Tap the switch (FaceTime) so it turns blue. Press on the line the small triangle. Then the "internal telephone directory" will appear. Choose which user you want to link to activities, Finish by pressing save on the top menu.