I know this is a long shot. Grandstream's documentation is "lacking". I'm unable to get BLF working on this phone. What happens: On holidays (like today) I use Call Flow Control to turn the phones off. The problem: I forget. I was trying to set up the BLF on one of the buttons to monitor ext 282 (CFC), but I can't get the damn thing to work. In asterisk, I have generate hints turned on and "Enable custom device states".
BLFs are troublesome indeed. I find myself working on them for weeks depending on the vendor. In the interim, you could also just add reminders to your installation - I find this more useful.