From the Granstream User Manual (Page 44):
Restore Factory Default Setting
WARNING: Restoring the Factory Default Setting will delete all configuration information of the phone.
Please backup or print all the settings before you restoring factory default settings. Grandstream is not
responsible for restoring lost parameters and cannot connect your device to your VoIP service provider.
INSTRUCTIONS FOR RESTORATION:
Step 1: Press "OK" button to bring up the keypad configuration menu, select "Config", press "OK" to
enter submenu, select "Factory Reset" (Please refer to Table 5-1 of keypad flow chart)
Step 2: Enter the MAC address printed on the bottom of the sticker. Please use the following mapping:
A: 22 (press the "2" key twice, "A" will show on the LCD)
D: 33 (press the "3" key twice, "D" will show on the LCD)
Example: if the MAC address is 000b8200e395, it should be key in as "0002228200333
NOTE: If there are digits like "22" in the MAC, you need to type "2" then press "->" right arrow key to
move the cursor or wait for 4 seconds to continue to key in another "2".
Step 3: Press the "OK" button to move the cursor to "OK". Press "OK" button again to confirm. If the
MAC address is correct, the phone will reboot. Otherwise, it will exit to previous keypad menu interface.