If your phone is receiving ghost/spam incoming calls you can enable a couple features that should stop them from reaching the phone.

Login into the web interface of the phone by entering the IP of the phone into a web browser. Default credentials should be admin/admin.

Navigate to Identity -> Security Settings:

Validate Incoming Messages: Yes
Check SIP User ID for Incoming INVITE: Yes
Authenticate Incoming INVITE: Yes

Apply and save the changes. The ghost calls should no longer be able to ring your phone if everything was done successfully.