Hi Paul,

There is a Setting called "Always Use Prime Line" if this is set to True, the Handset will only pick up calls ringing in on the Prime Line. If it is set to False, picking up the handset will Pickup any ringing call. It sounds like your setup has this set to True.

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 True 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