What you are looking for is called Enhanced Line Mode;
Enhanced Line Mode
The buttons on the left and right of the phone screen can be set up as line buttons, giving up to five more line keys. Users see an alert for an incoming call.
Enhanced line mode is supported on Cisco Unified Communications Manager 9.1(2), 10.5(2), and 11.0(1) and any higher releases that are generally available. The latest Device Package is required. You can set the line mode in the Product Specific Configuration Layout portion of the Device Configuration window in Cisco Unified Communications Manager.