Lighting specific configurations
If set to Y the record (e.g. contact) is always shown when making an outbound call.
After clicking on the phone number in a list view or any other screen, the related record like contact is loaded.
The advantage is that the correspondent record details are shown. The disadvantage is that the screen changes e.g. from opportunity to contact.
If set to N the screen will not change.
Also see Lightning setup for additional configuration settings