Each call coverage button is associated with another user on your telephone system. The button allows you to be alerted when that user has an unanswered call still ringing.
You can view the details of the call and answer it if required.
The settings of the user being covered control how long it rings at their phone before it also starts alerting on your call coverage button for that user.
Call coverage appearance buttons are not supported between users on different systems in a network.