Hi Scott,
I am thinking that you are referring to a CME installation, if that is the case you can set up Monitor Line Buttons that will do exactly what you are looking for.Check out this info:
Using a Monitor-Line Button for Speed Dial
For Cisco CME 3.2 and later versions, a monitor-line button can be used to speed-dial the monitor line's number. A monitor line is a line that is shared by two people. Only one person can make and receive calls on the shared line at a time, while the other person, whose line is in monitor mode, is able to see that the line is in use. Speed dialing is available when monitor lines' lamps are off, indicating that the line is not in use. For example, an assistant who wants to talk with a manager can press an unlit monitor-line button to speed-dial the manager's number.
A monitor-line lamp can be off or unlit only when its line is in the idle call state. The idle state occurs before a call is made and after a call has been completed. For all other call states, the monitor line lamp is on or lit.
The following example shows a monitor-line configuration. Extension 2311 is the manager's line, and ephone 1 is the manager's phone. The manager's assistant monitors extension 2311 on button 2 of ephone 2. When the manager is on the line, the lamp is lit on the assistant's phone. If the lamp is not lit, the assistant can speed-dial the manager by pressing button 2.
ephone-dn 11
number 2311
ephone-dn 22
number 2322
ephone 1
button 1:11
ephone 2
button 1:22 2m11
From this doc:
http://www.cisco.com/en/US/products/sw/voicesw/ps4625/products_configuration_guide_chapter09186a00805abb9e.html#wp1095438
Hope this helps!
Rob
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