The BLF only at paid version only. Free version is not supported.
Actually, the indicator not 100% accurate at this time. I'm not sure that this issue from 3CX or Grandstream. For the stable one snom also not 100% accurate as well.
Your Grandstream's FW version should be always hang/self-reboot during BLF enable. Using latest FW version 1.1.6.16 should be better. At last not self-reboot always.