See below for the section that applies to your phone model.
Current Grandstream models such as the GXP2130, GXP2140, GXP2160, and GXP2170:
Press the center button on the cursor key pad to bring up the menu.
Press 6 or navigate to "Phone" then press select.
Scroll down to "Call features" then press select.
Choose the line you want to forward then press select.
Enter the number to forward to, then press the "Enable" softkey. Press the down arrow key, and press the "Save" softkey. Your phone is now forwarded, and you should see this on the display:
When entering it, the number should be formatted exactly as you would dial it manually. Typically a 3-4 digit extension or a 10-digit outside phone number. This saves your forward-to number permanently (until you change or remove it). You can now enable/disable forwarding very easily from the main screen without entering the number again.
Older Grandstream models such as the GXP2100, GXP2124, and GXP1xxx lines:
Press the center button on the cursor key pad to bring up the menu.
Scroll down to "Call features" then press select.
Choose the line you want to forward then press select.
Select the type of forwarding, typically "Forward all."
Enter the number to forward to, then press the "Enable" softkey. The number should be formatted exactly as you would dial it manually. Typically a 3-4 digit extension or a ten digit outside phone number. This saves your forward-to number permanently (until you change or remove it). You can now exit the menus.
To remove forwarding, follow the steps above but select the "Disable" softkey to turn it off.