Step 1 - Access the Web GUI for the phones - You can do this by logging onto the IP address of the phones, on the GXP21xx series you can press the up arrow to get the IP address, or press the "NextScr" button on the GXP16xx series. You can also use Advanced IP scanner to get the IP address. Once you have the IP address, you can type this into a browser to access the phone. Once you’re here, you’ll need to log in and the default username is admin and the default password is also admin.
Step 2 - Under Accounts (Account 1 is the default), go to SIP Settings > Security Settings.
Step 3 - Enable the option for “Accept Incoming from SIP Proxy Only". This then should stop any ghost calls coming through to your Grandstream!