You can use BLF to monitor a specific extension for status changes on the phone. The BLF key illuminates solid red when the specific externsion is in use. You can tap a BLF key to dial out the monitored phone number when the monitored extension is idle. You can also pick up calls that are received on the monitored extension.
To configure BLF key via configuration template, you can refer to this Management configuration template
When you complete the configuration: