The telephone icon shown below is used as a 'Do Not Call' button. You can use this when a Contact has asked you to stop contacting them.
This function is available in the Property and Contact modules and should be used on both records directly.
Simply clicking the icon will turn it Red, signifying the functionality has been toggled ON. Turning on Do Not Contact will exclude the Contact from any Mail, SMS Merges, Newsletters or Auto-Update functionality created that includes the Contact record.