I have Grandstream GXP2160s and after some module updates the blf keys stop working. After rebooting the phones it works again.
OI found this at http://wiki.freepbx.org/display/FPG/EPM-Known+Limitations#EPM-KnownLimitations-Grandstream. However the BLFS do work on line keys
BLF’s on line keys do not work or update. They do get set in the gui. This is a problem with Grandstream firmware
GXW-42xx Time zone does not provision
Cannot set the text on line keys for Grandstream phones.
As of version 13.0.57, GXP-2130/35/40/60/70 phones use configuration entries that only work with Grandstream firmware 22.214.171.124 and above. Please upgrade your firmware to avoid endless reboots.
To fix endless reboots
In Firmware Management, move 1.13 or higher into the slot used by the template of the extension that is rebooting endlessly.
Remove the extension from extension mapping. This will stop the endless reboots and allow the phone to update the firmware.
Once the firmware is above 126.96.36.199, you can re-map the extension and the phone should be able to boot normally.