The receptionist has a Cisco 7965 phone with 24-button side-car configuration. The side-car button of the phone does not light up when the line is in use. Which phone configuration is missing?

The Subscriber Calling Search Space (CSS) in CUCM defines which partitions a BLF speed-dial button is permitted to subscribe to for presence information. Without this CSS configured on the BLF SD entry, the CTI subscription request to the monitored DN is rejected and the LED never illuminates when the line is in use.

Location is a Call Admission Control setting used to restrict bandwidth between sites and has no effect on whether a BLF LED illuminates.

BLF audible alert setting controls whether an audible tone sounds when a monitored line changes state, but does not affect the visual LED indicator on the side-car button.

Phone button template defines which button types are available on the phone chassis; if the side-car buttons are already provisioned and visible, the template is not the missing element preventing BLF illumination.