Hi Michael,
There is a setting in CUCM that dictates this behaviour called Always use Prime Line;
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.
Cheers!
Rob
