Those instructions apply to Cisco SPA 303, 504 and 525 models.
Toggling "Block Anonymous Calls" On / Off
Press the Setup button.
Scroll to Preferences and press the Select softkey.
Scroll to Block Anonymous Call and press the Edit softkey.
Press the y/n softkey to choose yes.
Press the Ok softkey.
Press the Save softkey.
Note that this feature will prevent anonymous calls from ringing the desk phones, however, those calls will immediately go to corresponding desk phones' Voicemail Box, so