Hi Daniel,
Just wanted to add a note to Brandons great info. If you only want to change the behaviour of being able to Pick-up the 5xxx extensions when ringing (without pressing the associated line button)Set the Always Use Prime Line parameter to False in Cisco CallManager. Because this parameter is clusterwide, any change affects all the phones. There is no interruption to the call handling and no need to restart any service or reboot the Cisco CallManager server.
In order to change this parameter, log in to the Cisco CallManager Administration page and perform these steps:
From the Service menu, choose Service Parameters.
Choose Publisher CallManager Server > Cisco CallManager service.
Under the Clusterwide Parameters (Device - Phone) section, scroll down to the Always Use Prime Line parameter, choose False for this parameter, and click Update.
Note: The default value for the Always Use Prime Line parameter is False. If the flag is set to True, when the phone goes off-hook or the speaker button is pressed, then the primary line is chosen and becomes the active line. If a call comes in on the second line of a user, going off-hook makes only the first line active. In this case, the user must select the second line in order to answer the call. If the flag is set to False, the phone automatically chooses a line as the active line based on the line status.
From this doc;
http://www.cisco.com/en/US/products/hw/phones/ps379/products_qanda_item09186a00800a6763.shtml#q39
Hope this helps!
Rob
