CME, I believe provides this functionality. Sidecars can exhibit Shared line status, and speed dial for the same button.
Monitor Lamp and Direct Station Select
For multibutton phones and expansion modules, the buttons for lines that are shared with other phones can be designated as monitor buttons. A monitor button is created primarily to show the status of a line that also appears on another phone. When the shared line is in use on the other phone, the console display for the line's monitor button shows an icon of a phone covered by an X. An idle line is represented by an icon of a phone with the handset in the on-hook position. Monitor buttons cannot be used to make outgoing calls, as opposed to line buttons for ordinary shared lines. This functionality is known on private branch exchange (PBXs) as a monitor lamp.
When a monitored line is idle, pressing the monitor button will speed-dial the monitored line. This functionality is sometimes known as fast transfer or direct station select.
The monitor lamp feature is configured using the m keyword of the button command. This feature can be configured on any multibutton IP phone, such as a Cisco IP Phone 7960 with one or two Cisco IP Phone 7914 modules. Phones are configured with a shared line for each extension that uses the monitor function.
