Difference Between a Line and a Button

Note that a line is similar to, but not exactly the same as, a button on the phone. A line represents a phone’s capability to make a call connection, so each button that can make a call connection becomes a line. (For example, unoccupied buttons or speed-dial buttons are not lines.) Note also that a line is not the same as an ephone-dn. A button with overlaid ephone-dns is only one line, regardless of whether it has several ephone-dns (extension numbers) associated with it. In most cases an ephone’s line numbers do match its button numbers, but in a few cases they do not.

via Cisco Unified Communications Manager Express System Administrator Guide – Configuring Headset Auto-Answer  [Cisco Unified Communications Manager Express] – Cisco Systems.