"BLF" stands for "Busy Lamp Field", which means that a light on the telephone's line button
tells you whether another extension is in use.
For this option to work, you must enable the Presence and Auto provisioning options in the extension. After that, you can turn on the BLF option under enhanced services and add extensions that you want to monitor.

Supported phones are:

• Linksys SPA962
• Polycom IP301
• Polycom IP501
• Polycom IP601
• Aastra 53i
• Aastra 55i
• Aastra 57i