Many VOIP based phone scams are originating outside of United States. The scammers target people posing to be IRS or from other federal agencies. Most of these seem to be skype or other VOIP based individuals or even call centers.
"Unknown caller" or "blocked calls" features can't stop them because these calls are from US phone numbers.The callers use legitimate US numbers and as a personal experience, they even answer the calls as a federal officer.
Hundreds (if not thousands) of honest citizens are being scammed every day and IRS and other federal agencies have flagged them as known phishing scams.
We should require phone companies to check the legitimacy of US Phone numbers being issued to US persons or businesses only. This will help in preventing phone scams originating overseas