iOS 11 Ringtone for No Caller ID
The following is an old trick, I don't know if it's still working :


Create a new contact called something like Non-Caller or No-Caller
Enter 000-000-0000 for the phone number.
Assign your unique ringtone or/and block it


src: https://discussions.apple.com/thread/6040239

Personally, I'm using the Truecaller app:

https://itunes.apple.com/us/app/truecaller/id448142450?mt=8