You configured the BLF keys but the BLF LED is not lightened when the monitor extension is registered or has incoming call or during in a call.
1. First of all, make sure the Monitored Account you used is registered successfully.
2. If your server is not Broadsoft platform, please don’t choose “Server Type” as “Broadsoft” please just choose “Default” otherwise it will make the BLF don’t work.
The path:
Go to Account->Advanced page.
3. Make sure the Line you choose for the BLF key is the one registered with same PBX as Monitored Account:
Path:
Go to DSSKey page 
4. And please note that we have an option for you to enable or disable BLF LED when the monitor extension is in idle, details are like below,
Path : go to Features->General Information
If this option “ LED Off in Idle” is configured with “Disabled”, when the monitor extension is in idle, the BLF LED will be lighten (Green);If this option is configured “ Enabled “ when the monitor extension is in idle, the BLF LED will not be lighten(off).
If the issue still exists, please reproduce it and get the Diagnostic File (config.bin, pcap file, level 6 syslog file)
Please refer to http://support.yealink.com/faq/faqInfo?id=707
For more information of “BLF” feature, please refer to Yealink Admin Guide:
Yealink SIP-T2 Series T4 Series T5 Series CP920 IP Phones Administrator Guide V85.1
If there are any other questions, please feel free to contact with ticket.yealink.com
