after doing some testing, i know that using the yealink firmware 1.13 on t4x phones is bad. the blf function does not work. rolling back to 1.12 fixes the problem. we use the blf feature to monitor some parking lot slots. with firmware 1.13, the lights do not light up.
i tested this on
FreePBX 13.0.153
PBX Firmware: 10.13.66-13
Current Asterisk Version: 13.9.1
Are you using CHAN_SIP or PJSIP?
the systems i tested this on were using chan_sip
Looks like Yealink has some bug fixes related to BLF in their latest firmware. x.80.0.130. Not sure if it is related to your issue but may be worth a try 
This is from the release notes…
“Fixed the issue that you configure a BLF key to monitor a specific user successfully,
and then the phone loose the network; but after the network is re-connected, the
BLF feature cannot work properly as before.”