This applies to home customers or a single extensions on a business account.
First, log into the extension portal (email firstname.lastname@example.org if you dont have the credentials)
Second, open the "Settings" folder on the left
Third, click on "Call Blocking"
Forth, click "Enable", Calls with CallerID "allow", Calls without callerID "block", then pick the result you want from the last 4 entries.